General Physiology: An Outline of the Science of Life

viduals apply themselves parallel to one another at their mouth - openings (Fig. 82, 1., o), their masses of protoplasm join together to form a bridge, and very characteristic changes in the nuclei begin. As above remarked, ciliate Infusoria have two forms of nucleus — •a macronucleus, or chief nucleus, and one or more micronuclei, or FIG. 82. — Conjugation of Paramcecium in the various successive stages ; A*, macronucleus ; n k, micronucleus. I. Beginning of conjugation. II. The micronucleus has divided twice in succession. III. Three of the four portions of the micronucleus perish, the fourth divides once more into a male m and a female w nucleus. IV. While the macronucleus is disintegrating, the two male nuclei, 1m and 5m, become exchanged and unite with the two female nuclei into a nucleus, V. t, which divides in turn into t' and t". VI. t' and t" divide. VII. From this division arise the rudiments of the new macronucleus pt and the new micronucleus nk'. The old macronucleus perishes. (After R. Hertwig.)

accessory nuclei. During conjugation the macronucleus perishes, disintegrating and dissolving in the protoplasm. If the Paramcecium be a form possessing one micronucleus, such as Paramcecium caudatum, where the relations are simplest, the micronucleus in each individual divides twice in succession, so that four partial nuclei arise. Three of these likewise dissolve in the protoplasm, but the fourth divides once more in each individual, and one half (the " male " nucleus) passes over the protoplasmic bridge into the other individual, so that each one of the pair now contains a " female " nucleus of its own, and a " male " nucleus from the other. These two nuclei immediately fuse together and then divide, one half becoming a new macronucleus, and the other half a new micronucleus. After such a mutual exchange of half-nuclei, the pair separate again and the conjugation is ended.

The phenomena of fertilisation in sexual reproduction are derived phylogenetically from the conjugation of asexual unicellular FIG. 83. — Fertilisation of the ovum of the thread-worm (Ascaris megdlocephala) in six successive stages. The maturation of the ovum, i.e., the extrusion of the polar bodies, takes place simultaneously. (After O. Hertwig.) organisms ; essentially the same facts are found in the former as in the latter. The process of fertilisation is not entirely the same in different species ; at least in the two species that thus far have been most fully investigated, the egg of the sea-urchin and that of the thread-worm of the horse, some slight differences have been observed, although the essential factors agree throughout.

We shall consider, first, the fertilisation of the ovum of the threadworm. The maturation of the ovum, i.e., the extrusion of the polar bodies, takes place while the sperm-cell is entering the egg. While the latter process is taking place (Fig. 83, 7), the eggnucleus, which up to this time has lain in the middle of the egg> wanders to the surface (Fig. 83, 77), where it divides twice in succession and gives off the polar bodies (Fig. 83, III and IV). In the meantime, the protoplasm of the sperm-cell has fused with the protoplasm of the egg-cell and withdrawn from further observation. The sperm-nucleus, however, has wandered into the middle of the egg. to which place also the egg-nucleus, after giving off the polar bodies, returns from the periphery. The two nuclei now apply themselves to one another, surround themselves with a transparent envelope, and show distinctly two large chromatic loops in each. At the same time, two centrosomes become visible and begin to surround themselves at opposite sides of the nuclei with a circle of rays (Fig. 83, V). In the thread-worm the nuclear substances do not fuse, but the well-known spindle of nuclear division develops, beginning at the two centrosomes, and the spindle-fibres on either side draw to their respective poles one chromatic loop from the egg-nucleus and one from the spermnucleus, so that each half of the egg-cell obtains one nuclear component from the egg and one from the spermatozoon (Fig. 83, VJ\ The fertilisation is thus ended, and at the same time the first division of the ovum is prepared for ; the latter now proceeds in the usual manner, the egg being constricted through the equator of the spindle, while the nuclei in the two halves assume their resting-form.

As regards individual points, the fertilisation of the egg of the sea-urchin proceeds somewhat differently. The maturation of the ovum is completely ended when the spermatozoon enters. Further, the egg- and the sperm-nuclei fuse completely into a single nucleus before the division into the first two cleavage-cells of the ovum takes place. Fol ('91) supposed that he had made in the further course of the fertilisation -process an observation of special interest, because it appeared to shed some light upon the behaviour of the centrosome. What he saw was the following : With the sperm-cell, a sperm-centrosome enters the ovum, which still possesses, in addition, its own centrosome. After the union of egg-nucleus and sperm-nucleus the two centrosomes come to lie at the two opposite poles of the common nucleus, which is surrounded by a simple protoplasmic radiation. Each of the two centrosomes thereupon divides, constricting itself like a dumb-bell into two, each of which wanders across to the other of the opposite side, a phenomenon that was termed by Fol the " quadrille of the centrosomes." Thus, each half of the original egg-centrosome comes into union with one half of the sperm -centrosome and finally fuses with it, so that only two centrosomes are present again at the opposite poles of the nucleus ; each of these two, however, consists in half of the substance of the egg-centrosome and in half of that of the sperm-centrosome. These two centrosomes now form the poles for the following division of the nucleus

and become surrounded each with its own protoplasmic radiation. Thus the fertilisation is ended and the division of the fertilised ovum into the first two cleavage-cells is introduced. But, unfortunately, this account by Fol concerning the course of fertilisation and the much-quoted " quadrille of the centrosomes " appears to rest upon incorrect observation. At least Boveri ('95) and, in harmony with him, Wilson and Mathews ('95) in sea-urchin eggs, and Mead ('95) in the eggs of tube-worms (Chcetopterus pergamentaceus), have found that such a quadrille of the centrosomes does not exist, that rather the centrosome of the egg-cell perishes and disappears (Mead) without playing any rdle, while that of the sperm-cell after fertilisation divides alone in the egg-cell into two centrosomes, each of which becomes a centre for the protoplasmic radiation and the succeeding division of the fertilised ovum.

A re'sume' of the essential factors of the phenomena of fertilisation leads to the following statement : Fertilisation consists in the union of two cells, the egg-cell and the sperm-cell, in which protoplasm fuses ivith protoplasm and nucleus with nucleus ; thus, in the succeeding division of the fertilised egg-cell each half obtains material from both the fused cells, and from both the protoplasm and the nucleus. Development may be defined in a general sense as a continuous series of changes. If we leave out of consideration the reproduction of the multicellular organism by the constriction of entire parts of the body, as in gemmation and fission, where the essential cell-groups of the individual systems of organs are transferred directly from the parent organism to the buds or products of fission, the formation of the multicellular organism consists only in its development from the egg-cell. The multicellular organism develops gradually from a single cell, whether the egg develops without fertilisation, as in the interestingphenomenon of parthenogenesis (which occurs in certain lower animals and affords a real background for the ancient legend of the immaculate conception), or whether the egg has previously been fertilised, as is the general rule in the development of animals and plants.

Development is present in unicellular organisms, but here the whole cycle proceeds in a single cell. The development of the Protista forms an interesting analogy to that of multicellular organisms, both animals and plants. In the lowest forms, such as Amceba, development is identical with simple growth. An Amoeba changes simply by increasing in mass and then dividing. The halves then grow again until they become so large that they again divide. The whole developmental cycle of Amceba consists in growth up to cell-division. We see, therefore, that growth and

cell-division are the simplest elements that development demands ; in fact, in the whole living world there is no development without growth and cell-division. In all Protista that reproduce by sporeformation, there occurs a development expressing itself in complex changes of form. In this case the spores, which are totally unlike the mother-cell, must pass through a series of changes of form until they become like it. The development of the Protista has been little studied. Nevertheless, Rhumbler ('88) has followed completely and with great care that of the infusorian genus Colpoda. Colpoda is a small bean-shaped infusorian, the surface of whose whole body is ciliated (Fig. 84, A). In spore-

FIG. 84.— Development of Colpoda cucullus. (After Rhumbler.) formation the body surrounds itself with a thick envelope or cyst (£), within which by giving off water the body constantly diminishes its volume. Finally it extrudes all undigested foodparticles and draws itself together into a ball ((7), which loses its cilia and surrounds itself by a second smaller envelope (D). The contents of this second envelope (E) break up into single spores, which together with a remnant consisting of useless material burst the capsule and freely wander out (F). From each spore (£) a new individual develops by the spore transforming itself into a small amoaba-like being which creeps about, takes food, grows (H, J, K, L\ develops a long flagellum with which it swims (M), and finally contracts into a small spherical cell (N), which covers its surface with cilia (0), and by further growth gradually assumes the form of a Colpoda (P, Q, R). Thus the developmental cycle is completed.

That which comes to pass among the Protista in a single cell, takes place in an aggregate of cells in the development of the multicellular organism. In accordance with the above considerations concerning reproduction, the development of the multicellular organism from the unicellular egg can take place by continued celldivision only. But in this process two factors play important rdles : first, the products of the division of the egg-cell do not separate as in most Protista, but remain in connection with one another; and, second, the products of division are not always alike, but by unequal division two forms of cell, wholly different from each other and from the mother-cell, can arise. In this manner is

FIG. 85. — A. Eudorina elegans, B. Magosphcera planula, two multicellular organisms consisting of similar cells. (After Haeckel.) rendered possible the origin, not only of a multicellular organism, but of such an organism with differentiation of various kinds of tissues and organs. If the first factor alone were present, there would result a cell-community consisting of many cells, all of which, however, would be alike. Such organisms exist among Protista (Fig. 85), and are regarded as cell-colonies that have a republican constitution, i.e., in which every cell is exactly like every other. These forms are the intermediate links between the really unicellular organisms and the animals or plants. In the bodies of animals and plants, even the lowest, the cells are not all alike, and this differentiation, through which alone the development of a complex cell-community becomes possible, depends upon the efficiency of the second factor, unequal celldivision. Hence, cell-division, both equal and unequal, and cohesion of the cells are the factors that bring about the development of a differentiated cell-community.

We cannot go further into the special phenomena of the individual development of animals and plants, and must refer the reader to the detailed works of Haeckel ('91), 0. Hertwig ('90), and Korschelt and Heider ('90), who treat embryology as an independent science. We must, nevertheless, glance at that important law which, as has already been seen, prescribes a definite path to individual development, namely, the fundamental law of biogenesis. Karl Ernst von Baer, the founder of embryology, discovered that in the embryonic development of widely different forms of animals, stages occur that appear strikingly similar; and after Darwin's epoch-making labour Fritz Miiller ('64) expressed clearly the fact that the developmental history of the individual is a short repetition of the whole course of development which the corresponding species has undergone during the development of the earth. It was Haeckel's service to formulate more exactly the fundamental law of biogenesis and emphasise the existence of a causal relation between ontogeny and phylogeny. Haeckel ('66) showed that individual development, or ontogeny, is only in gross outline a repetition or palingeny of the racial development or phylogeny, but that this repetition is frequently blurred or falsified by the appearance of phenomena that are not present in the phylogeny of the corresponding form and which, therefore, he termed the phenomena of falsified development or cenogeny. Hence, in the individual development of every organism, two elements may be distinguished : first, the palingenetic phenomena, which recapitulate in brief the racial development of the form in question, and, second, the cenogenetic phenomena which have arisen supplementarily by adaptation and have altered and blurred the course of the palingenetic phenomena.

The causal explanation of these facts lies in the two factors which, as has been seen, control the whole development of organic life, namely, heredity, which maintains form, and adaptation, which changes it. The characteristics of an organism comprise more than those which it shows at any single moment of its development or as an adult animal. To them belong the whole sum of peculiarities and changes which it has shown from its simplest beginnings ; for the later characteristics do not represent anything new and spontaneous, but proceed immediately and continuously from the earlier ones. If, therefore, heredity conveys the characteristics of the parents to the offspring, it must convey to the latter, not only the characteristics possessed by the parents at the moment of the production of the offspring, but the whole sum of parental characteristics, and among them those that the parents have shown during their development. Hence the peculiar course of development that the parents have gone through must be transmitted to the children, and the latter must

go through the same development. Since this is true of every generation of parents and children, it must be true also of all the ancestors of the race, even the earliest, i.e., the children are the historic product of the whole racial development and in their developmental history must pass through the whole history of the race. But this is true only on the condition that heredity is the sole factor that determines form. In such a case every minute peculiarity that was once present in the ancestral series of the organism would repeat itself with painful exactness in the development of the latter. Since individual development demands a relatively short time and racial development shows an inconceivable variety of changes in form, the remarkable spectacle would be presented of the ontogeny of a higher animal appearing like the picture in a constantly turning kaleidoscope, which never remains the same but presents to the eyes at every moment a different form. It is well known that this is not the case, but that the racial development is recapitulated only in bare outlines and undergoes manifold changes ; these latter are the cenogenetic phenomena, which are caused by the second factor that determines form, namely, adaptation. It has been seen that the form of every organism is determined in a certain degree by external conditions. Any form that lived at a certain geological period in the racial series of an animal is, therefore, determined among other things by the conditions that prevailed upon the earth's surface at that period. The conditions now are entirely different. But not only have the conditions upon the earth become different, but the animal in its development is under wholly different conditions from the completed animal, especially if the first developmental stages are passed through within the mother's body. Since, however, these external conditions must effect an adaptation of the organism in question, it is explained why in the ontogenetic recapitulation of the phylogenetic series there appears not only a simplification but also an alteration of certain phenomena.

Simplification comes about because developmental stages which at the time of their appearance represented special adaptations to certain conditions become bred out as useless and disturbing factors now when those conditions are wanting; alteration occurs by the adaptation of certain developmental stages themselves to the new conditions. It is clear that here also selection controls the change of form, and that characteristics arising cenogenetically become transmitted like original ones.

Accordingly, with Haeckel (75), the fundamental law of biogenesis may be formulated in brief as follows : " Germinal development is an epitome of racial development ; the more complete, the more the abridged development is maintained ly heredity ; the less complete, the more a falsified development is introduced ~by adaptation. For a long time natural science has distinguished different forces which bring about the phenomena of motion in nature. In the scientific sense force is nothing but an expression for the cause of motion, for we know nothing concerning it except that it causes motion. Sense-perception is not force, it is merely motion. Accordingly, since early times, wherever different kinds of motion have been seen, different kinds of force have been assumed. It thus came about in time that a large number of forces were distinguished, which could not in any way be compared with one another, because some kinds were only special cases of others, some were combinations of several kinds, and some were not forces at all. The force of gravity, muscular force, and the force of will were all spoken of. This condition of things has not yet wholly disappeared. The forces that physics still recognises are not equivalent things, and little light has been thrown, even yet, upon the relations of certain ones to others.

In recent times, in accordance with the usage of Th. Young and Thomson, the old and easily misunderstood name " force " has been replaced by the term " energy," and what earlier were termed different forces are termed now different forms of energy. Thus, physics now recognises in general the following forms of energy : 1. Chemical energy (chemical affinity, attraction of atoms). 2. Molecular energy (cohesion, adhesion, attraction of molecules). Modern natural science, as is well known, conceives the physical world to be composed of extremely small particles ; it terms the particles that cannot be divided further without losing their properties, molecules, and those that compose the molecule and are indivisible, atoms. Chemical energy is that form of energy by which atoms attract one another in order to form a molecule ; molecular energy that form by which molecules attract one another in order to form masses. If a mass is in motion and strikes against another movable body, it puts this likewise into motion if the impact be strong enough. The form of energy that puts in motion the body that is struck is mechanical energy. Further, masses attract one another,

like the atoms in the molecule and the molecules in the mass ; since Newton's immortal discovery it has been known that the paths of the heavenly bodies result from the mutual attractions of their powerful masses. This mass attraction, which binds the earth to the sun, and the moon to the earth, and compels a stone thrown upward to return again to the earth, is gravity or the energy of gravitation. Finally, thermal, photic, electrical, and magnetic energy are the forms of energy that put the atoms of the hypothetical ether, which fills universal space and penetrates all bodies, into those forms of motion termed heat, light, electricity and magnetism ; for in accordance with the researches of modern physics the phenomena of heat, light, electricity and magnetism result merely from the vibrations of very minute particles.

But simple reflection shows that these forms of energy are not equivalent and separate. If all matter, including the hypothetical ether, is composed of atoms as its smallest physical particles, and if nothing corporeal exists beyond matter, all forms of energy, since they are associated with matter, must have their seat in atoms. In other words, atoms are the smallest particles endowed with energy, and it is evident that the forms of energy that are assumed for the motions of masses, such as gravity, must have their seat in atoms. Now, a priori, it is in the highest degree improbable that every atom is provided with eight different forms of energy. Scientific experience, which shows that everywhere in nature apparent multiplicity can be traced to unity, suggests that all these different forms of energy may be traced to a single form. As a matter of fact, molecular and mechanical energy and energy of gravitation, upon the one side, have been put into close relations with one another, as well as thermal, photic, electrical and magnetic energy upon the other side ; and very recently electro-chemical researches have made it appear that a very close relation exists between chemical and electrical energy. Hence we have a wellfounded hope that before very long physics will succeed in demonstrating all forms of energy to be merely the expression of one and the same form, which appears different under different conditions ; just as chemistry hopes to be able sometime to reduce the multiplicity of the chemical elements to the properties of a single original element, perhaps the universal ether.

The probability that the different forms of energy are only different modes of appearance of one and the same energy, amounts almost to a certainty in the light of the fact that one form of energy may be changed into another form, and in nature is continually so changed. As is well known, this all-important fact finds expression in the law of the conservation of energy, which was discovered and founded by Robert Mayer and Helmholtz, and which has become the foundation of our whole modern view of nature. This fact is explicable only in accordance with the idea that energy

itself is always the same, and that the different forms of its appearance are merely cloaks, which may be exchanged according to the conditions at the moment. Just as we speak of different forms of energy, we can distinguish in the single form two different modifications, according as the energy actually produces motion or only has potentially the capacity of putting into action under proper conditions. Physicists term these two modifications kinetic energy (also actual energy, or energy of motion) and potential energy (energy of position). The energy of gravitation, e.g., is kinetic when it draws a stone to the earth at the moment when the stone is set free ; it is potential so long as the stone is fixed above the earth's surface. Likewise, chemical energy is kinetic when it brings two atoms to each other ; but it is potential when an atom has no other one in its vicinity that it can attract. Kinetic energy passes over constantly into potential energy and vice versa.

The law of the conservation of energy, therefore, controls all that happens in nature; it is the fundamental law of energetics. According to it, as has already been seen, energy in the world never originates or disappears ; the sum of energy in the world is constant, just as the law of the conservation of matter expresses the same constancy in the quantity of matter. Where a certain quantity of energy seems to originate or disappear, in reality it simply goes over into another form or modification. If, e.g., an electric current be passed through a vessel containing water, the electrical energy seems to be lost. But in reality it does not go out of existence, for it has been seen that the molecules of the water are decomposed into their hydrogen and oxygen atoms, and these accumulate in a gaseous state upon the two poles of the electrical conductors. Hence the electric current has performed work and has separated the atoms of the molecules of water from one another. But the atoms of hydrogen and oxygen set free have a chemical affinity for one another ; hence in the experiment the kinetic energy of the electric current has simply been transformed into the potential energy of chemical affinity. If, therefore, the separate atoms of hydrogen and oxygen be brought again into union under proper conditions, the chemical potential passes over again into kinetic energy, and a certain quantity of heat is liberated thereby. This heat can be transformed again into electricity in a thermo-electric apparatus, and, if the technical difficulties would allow the whole experiment to be carried out with sufficient exactness, it would be found that the same quantity of electricity has again been obtained as was consumed previously in the splitting-up of the water. During all transformations the original quantity of energy remains the same.

In order to have a unit for the measurement of any quantity of energy, physicists have chosen, in accordance with Joule's researches upon the relation of heat to mechanical energy, a certain quantity of heat as the unit of heat or calorie. A calorie is that quantity of heat that is necessary to warm one kilogram of water from 0° to 1° C. Heat was chosen with good reason as that form of energy which may serve as a unit of measure for all others, for it holds a peculiar position in relation to all others ; it is the sole form into which all others can be transformed completely. When, therefore, it is desired to express in numbers a quantity of any desired form of energy, e.g., mechanical or chemical energy, the latter is expressed in measures of heat, that is, in the number of equivalent calories. Thus, one calorie computed in the form of mechanical work corresponds to the quantity of energy that is needed to raise a weight of 424 kilograms one metre high ; in other words, the mechanical equivalent of one calorie is 424 kilogrammetres and, vice versa, one calorie is the heat-equivalent of 424 kilogrammetres. In the same way the quantity of all other forms of energy can be expressed in heat-equivalents. The calorie is the unit of measure for all energy.

Life has often been compared with fire, an idea which plays a rdle in the oldest mythological folk-views of nature and, as is well known, first assumed a fixed form in the philosophy of Heraclitus. In many points the comparison is fitting. To extend it somewhat further, the organism is the burning coal which is being constantly consumed, the breath is the smoke, and the food is the freshly added fuel which constantly replaces the old. Just as the burning mass of coal represents a physical system in which a continual transformation of energy is taking place, potential energy being introduced with the fuel and transformed into two forms of kinetic energy manifest outside, namely, heat and, by proper arrangement, as in the steam engine, mechanical work, so an organism is a physical system in which a similar transformation of energy continually takes place. Just as by heaping new coal upon the fire, energy is added in the potential form, so also, at least in the animal organism, by far the greater part of all the energy introduced is potential energy. The introduction of energy is considerably less evident to the eye than the production of energy; the latter results from the transformation of the introduced potential and is expressed in movements and other visible work.

Since cenfused ideas concerning the transformation of energy in chemical processes are wide-spread, it will be advantageous first to glance at the general facts. By chemical energy is understood, as is well known, the capacity of atoms to attract other atoms ; this property has also been termed chemical affinity. Every atom, regarded as isolated, represents accordingly a small magazine of energy. The chemical energy in it is potential so long as the atom has no opportunity to unite by means of its affinity with another atom. But, as soon as two atoms combine, a part of the potential, corresponding to the strength of their affinities, passes over into kinetic energy and is set free in the form of heat, light, mechanical energy, etc. Since, further, chemical affinity is quantitatively very different in different kinds of atoms, the stronger the combining affinities, the more energy is set free. A chemical compound, must, therefore, contain less potential energy, the stronger the affinities are that have brought together its atoms. Vice versa, if two combined atoms become separated, a certain quantity of kinetic energy is absorbed in the process, and after the separation the same quantity appears again in the potential form as the free affinities of the atoms. Thus there is a complete cycle.

An example will make this relation more evident. Suppose a strong glass cylinder to be inverted over a mercury trough and to contain in a small space free from mercury a gaseous mixture consisting of two-thirds hydrogen and one-third oxygen ; such a mixture consists of molecules whose atoms contain large quantities of potential energy in the form of chemical affinity for one another. If, now, the conditions be made such that the atoms of oxygen and hydrogen can combine, the atoms rush eagerly toward one another, unite and give off to the outside all their stored potential in the form of heat, light, and mechanical energy. A spark appears, the cylinder becomes heated, and the mercury is forcibly driven down. The latter soon rises again, for the vapour that results from the union of the atoms of oxygen and hydrogen becomes condensed with the increasing cooling into water, which finally occupies only a minute space within the cylinder. Thus, in the synthesis of water from hydrogen and oxygen the potential energy of chemical affinity is transformed into kinetic energy and is set free as heat, light, etc. Hence the molecule of water has lost to its environment this quantity of energy, and this can be exactly determined. Vice versa, the atoms of water can be separated again into atoms of hydrogen and oxygen by introducing from outside the same quantity of energy. Electrical energy serves best for this purpose. If an electric current be passed through water, atoms of hydrogen and oxygen are set free at the poles in the same degree as the electrical energy disappears. Hence energy is absorbed in separating the atoms of the watermolecule ; but this energy appears again as the potential of chemical affinity in the free atoms, for, when the free hydrogen and oxygen are brought into combination, kinetic energy is obtained anew, and so on.

This consideration is very important, for from it there follows a principle of far-reaching significance which usually is not formulated with sufficient clearness, viz. : In the combination of atoms kinetic energy is liberated ; in the separation of atoms kinetic energy is absorbed. This principle, which is a necessary sequence of the law of the conservation of energy, must be considered as a fundamental one for all chemical transformations, and forms the starting-point for an understanding of all the phenomena connected with the transformation of energy within the living organism. That as a rule it has not been established and applied with sufficient clearness, is to be ascribed chiefly to the fact that in certain cases at first sight it suffers apparently an exception. To make the relations clear, this must be considered, at least briefly.

To express in terms of heat the energy that is transformed in a chemical process, there are recognised processes in which heat is evolved and processes in which heat is absorbed. In accordance with the nomenclature of thermo-chemistry, the heat that is evolved in a chemical process is termed the positive thermo-chemical equivalent, the heat that is absorbed, on the other hand, the negative thermo-chemical equivalent. From the above considerations, it would be expected that all synthetic processes, i.e., all processes in which bodies unite, would be accompanied by an evolution of heat, for in every synthesis atoms become united, and in every union of atoms energy is liberated. Vice versa, it would be expected that all decomposition-processes, i.e., all processes in which united atoms become separated, would be accompanied by an absorption of heat. If the conceptions of synthesis and decomposition are employed in their pure significance, this is always the case. Nevertheless, at first sight there appear certain exceptions to the rule. For example, some syntheses are known in chemistry, such as that of hydrogen iodide, which are accompanied by an absorption of heat ; on the other hand, there are many decompositions, especially of the more complex compounds, such as nitroglycerine and other explosives, in which a powerful evolution of energy takes place. These are undeniable facts, but, if the details of these processes be analysed somewhat fully, the apparent paradox becomes at once clear and in reality confirms the law.

Since no free atoms are known, but since the similar atoms of every chemical element are united always into molecules, or groups of atoms, it is evident that unless whole molecules enter into combination without rearrangement of their atoms or are split off from a combination as preformed groups, then a decomposition of the active molecules into their atoms must precede every synthesis, and a synthesis of the free atoms into new molecules must follow every decomposition. Hence, no synthesis occurs without previous decomposition, and no decomposition without subsequent synthesis. Accordingly, it is clear that under certain circumstances heat can be absorbed in a synthesis : for example,