An indispensable condition of the circulation of matter (metabolism) which we call life is the physical process of osmosis, which is connected with the variations in the quantity of water in the living substance and its power of diffusion. The plasm, which is of a spongy or viscous consistency, can take in dissolved matter from without (endosmosis) and eject matter from within (exosmosis). This absorptive property (or "imbibition-energy") of the plasm is connected with the colloidal character of the albuminoids. As Graham has shown, we may divide all soluble substances into two groups in respect of their diosmosis—crystalloids and colloids. Crystalloids (such as soluble salt and sugar) pass more easily into water through a porous wall than colloids (such as albumen, glue, gum, caramel). Hence we can easily separate by dialysis two bodies of different groups which are mixed in a solution. For this we need a flat bottle with side walls of india-rubber and bottom of parchment. If we let this vessel float in a large one containing plenty of water, and pour a mixture of dissolved gum and sugar into the inner vessel, after a time nearly all the sugar passes through the parchment into the water, and an almost pure solution of gum remains in the bottle. This process of diffusion, or osmosis, plays a most important part in the life of all organisms; but it is by no means peculiar to the living substance, any more than the absorptive or viscous condition is. We may even have one and the same substance—either organic or inorganic—in both conditions, as crystal or as colloid. Albumen, which usually seems to be colloidal, forms hexagonal crystals in many plant-cells (for instance, in the aleuron-granules of the endosperm), and tetrahedric hœmoglobin-crystals in many animal-cells (as in the blood corpuscles of mammals). These albuminoid crystals are distinguished by their capacity for absorbing a considerable quantity of water without losing their shape. On the other hand, mineral silicon, which appears as quartz in an immense variety (more than one hundred and sixty) of crystalline forms, is capable in certain circumstances (as metasilicon) of becoming colloidal and forming jelly-like masses of glue. This fact is the more interesting because silicium behaves in other ways very like carbon, is quadrivalent like it, and forms very similar combinations. Amorphous (or non-crystalline) silicium (a brown powder) stands in relation to the black metallic silicon-crystals just as amorphous carbon does to graphite-crystals. There are other substances that may be either crystalloid or colloid in different circumstances. Hence, however important colloidal structure may be for the plasm and its metabolism, it can by no means be advanced as a distinctive feature of living matter.
Nor is it possible to assign an absolute distinction between the organic and the inorganic in respect of morphology any more than of chemistry. The instructive monera once more form a connecting bridge between the two realms. This is true both of the internal structure and the outward form of both classes of bodies—of their individuality (chapter vii.) and their type (chapter viii.). Inorganic crystals correspond morphologically to the simplest (unnucleated) forms of the organic cells. It is true that the great majority of organisms seem to be conspicuously different from inorganic bodies by the mere fact that they are made up of many different parts which they use as organs for definite purposes of life. But in the case of the monera there is no such organization. In the simplest cases (chromacea, bacteria) they are structureless, globular, discoid, or rod-shaped plasmic individuals, which accomplish their peculiar vital function (simple growth and subdivision) solely by means of their chemical constitution, or their invisible molecular structure.
The comparison of cells with crystals was made in 1838 by the founders of the cell-theory, Schleiden and Schwann. It has been much criticised by recent cytologists, and does not hold in all respects. Still it is of importance, as the crystal is the most perfect form of inorganic individuality, has a definite internal structure and outward form, and obtains these by a regular growth. The external form of crystals is prismatic, and bounded by straight surfaces which cut each other at certain angles. But the same form is seen in the skeletons of many of the protists, especially the flinty shells of the diatomes and radiolaria; their silicious coverings lend themselves to mathematical determination just as well as the inorganic crystals. Midway between the organic plasma-products and inorganic crystals we have the bio-crystals, which are formed by the united plastic action of the plasm and the mineral matter—for instance, the crystalline flint and chalk skeletons of many of the sponges, corals, etc. Further, by the orderly association of a number of crystals we get compound crystal groups, which may be compared to the communities of protists—for instance, the branching ice-flowers and ice-trees on the frozen window. To this regular external form of the crystal corresponds a definite internal structure which shows itself in their cleavage, their stratified build, their polar axes, etc.
If we do not restrict the term "life" to organisms properly so-called, and take it only as a function of plasm, we may speak in a broader sense of the life of crystals. This is seen especially in their growth, the phenomenon which Baer regarded as the chief character of all individual development. When a crystal is formed in a matrix, this is done by attracting homogeneous particles. When two different substances, A and B, are dissolved in a mixed and saturated solution, and a crystal of A is put in the mixture, only A is crystallized out of it, not B; on the other hand, if a crystal of B is put in, A remains in solution and B alone assumes the solid crystalline form. We may, in a certain sense, call this choice assimilation. In many crystals we can detect internally an interaction of their parts. When we cut off an angle in a forming crystal, the opposite angle is only imperfectly formed. A more important difference between the growth of crystals and monera is that the former only grow by apposition, or the deposit of fresh solid matter at their surface; while the monera grow, like all cells, by intussusception, or the taking of new matter into their interior. But this difference is easily explained by their difference in consistency, the crystal being solid and the plasm semi-fluid. Moreover, the difference is not absolute; there are intermediary stages between apposition and intussusception. A colloid globule suspended in a salt solution in which it is not dissolved may grow by intussusception.
It was once the custom to restrict sensation and movement to animals, but they are now recognized to be present in nearly all living matter. They are, in fact, not altogether lacking in crystals, as the molecules move in crystallization in definite directions, and unite according to fixed laws; they must, therefore, also possess sensation, as we could not otherwise understand the attraction of the homogeneous particles. We find in crystallization, as in every chemical process, certain movements which are unintelligible without sensation—unconscious sensation, of course. In this respect, also, then, the growth of all bodies follows the same laws (cf. chapters xiii. and xv.).
The growth of a crystal is restricted like the growth of a moneron or of any cell. If the limit is passed and the conditions remain favorable to growth, we find an instance of that excessive or transgressive growth which we call reproduction in the case of living individuals. But we find just the same kind of extension in the inorganic crystal. Every crystal grows in a supersaturated medium only up to a definite size, which is determined by its chemical-molecular constitution. When this limit is reached a number of small crystals appear on the large one. Ostwald, who has made a thorough comparison of the process of growth in crystals and monera, especially notices the striking analogy between a bacterium (a plasmophagous moneron) growing and multiplying in its nutritive fluid and a crystal in its matrix. When the water slowly evaporates from a supersaturated solution of Glauber-salt, not only does a crystal slowly grow in it, but several young crystals appear on it. The analogy with the bacterium multiplying in its nutritive fluid can even be followed as far as its permanent forms or "spores." This quiescent form is assumed by the bacterium if its supply of food is exhausted; if fresh food is added, the multiplication by cleavage begins again. In the same way the crystals of Glauber-salt begin to decay when the solution is evaporated; they lose their crystal water, but not their power of multiplication. Even the amorphous powder of the salt causes again the formation of new watery crystals when put in a supersaturated solution. But the powder loses this property when it is heated, just as the dormant forms (or spores) of the bacteria lose their power of germination.
The exhaustive comparison of the growth of crystals and monera (as the simplest forms of unnucleated cells) is important, because it shows the possibility of tracing the vital function of reproduction—which had usually been regarded as a quite special "wonder of life"—to purely physical conditions. The division of the growing individual into several young ones must necessarily take place when the natural limit of growth has been passed, and when the chemical composition of the growing body and the cohesion of its molecules allow no further enlargement by the assumption of new matter. In order to illustrate the limit of this transgressive growth by a simple physical example, Ostwald imagines a ball placed in a small flat basin, built up high on one side. The ball is in a state of equilibrium in the basin; when it is lightly pushed aside it always returns to its original position. But when the push goes beyond a certain point, and the ball is thrust over the side of the basin, the balance is lost; the ball does not return, but falls to the ground. The crystal behaves just in the same way in a supersaturated solution when it exercises its power of forming new crystals; and it is just the same with the bacterium growing in a nutritive fluid when it passes the limit of its volume of growth, and divides into two individuals.
As we can find no morphological and little physiological difference between the living and non-living, we must look upon metabolism as the chief characteristic of organic life. This process causes the conversion of food into plasm; it is determined by the vital force itself, and is the formation of new living matter. It thus effects the nutrition and growth of the living being, and therefore its reproduction, which is merely transgressive growth. As I shall describe this metabolism fully in the tenth chapter, I will do no more here than emphasize the fact that this vital process also has analogies in inorganic chemistry, in the curious process of catalysis, especially that form of it which we call fermentation.
The distinguished chemist Berzelius discovered in 1810 the remarkable fact that certain bodies, by their mere presence, apart from their chemical affinity, set other bodies in decomposition or composition without being themselves affected. Thus, for instance, sulphuric acid changes the starch in sugar without undergoing any alteration itself. Finely ground platinum brought in contact with hydrogen-superoxide divides it into hydrogen and oxygen. Berzelius called this process catalysis; Mitscherlich, who discovered the cause of it to be the peculiar surface-action of many bodies, gave it the name of "contact-action." It was afterwards discovered that catalysis of this kind is very general, and that a special form of it—fermentation—plays an important part in the life of organisms.
This special form of contact-action which we call fermentation is always effected by catalytic bodies of the albuminoid class, and, in fact, of the group of non-coagulable proteins which are known as peptones. They have—in however small a quantity—the capacity to throw into decomposition large masses of organic matter (in the form of yeast, putrid matter, etc.) without themselves taking part in the decomposition. When these ferments are free and unorganized they are called enzyma, in opposition to organized ferments (bacteria, yeast-fungi, etc.); though the catalytic action of the latter also consists essentially in the production of enzyma. The recent investigations of Verworn, Hofmeister, Ostwald, etc., have shown that these catalyses play everywhere an important part in the life of the plasm. Many recent chemists and physiologists are of opinion that plasm is a colloid catalysator, and that all the varied activities of life are connected with this fundamental vital chemistry. Thus Franz Hofmeister (1901) says in his excellent work on The Chemical Organization of the Cell: