instances as "hydrated silicic acid, liydrated alumina, and other metallic peroxides of the aluminous class, when they exist in the soluble form; with starch, dextrine and the gums, caramel, tannin, albumen, gelatine, vegetable and animal extractive matters.'* Describing the properties of colloids. Professor Graham says: — ^' Although often largely soluble in water, they are held in solution by a most feeble force. They appear singularly inert in the capacity of acids and bases, and in all the ordinary chemical relations." * * * "Although chemically inert in the ordinary sense, colloids possess a compensating activity of their own arising out of their physical properties. "While the rigidity of the crystalline structure shuts out external impressions, the softness of the gelatinous colloid partakes of fluidity, and enables the colloid to become a medium of liquid diffusion, like water itself." * * * « Hence a wide sensibility on the part of colloids to external agents. Another and eminently characteristic quality of colloids is their mutability." * * * << The solution of hydrated silicic acid, for instance, is easily obtained in a state of purity, but it cannot be preserved. It may remain fluid for days or weeks in a sealed tube, but is sure to gelatinize and become insoluble at last. Nor does the change of this colloid appear to stop at that point; for the mineral forms of silicic acid, deposited from water, such as flint, are often found to have passed, during the geological ages of their existence, from the vitreous or colloidal into the crystalline condition (H. Rose). The colloid is, in fact, a dynamical state of matter, the crystalloidal being the statical condition. The colloid possesses energia. It may be looked upon as the primary som^ce of the force appearing in the phenomena of vitality. To the gradual manner in which colloidal changes take place (for they always demand time as an element) may the characteristic protraction of chemicoorganic changes also be referred."
The class of colloids includes not only all those most complex nitrogeneous compounds characteristic of organic tissue, ORGANIC MA'ITEK. 17 and suiidiy of the oxy-hydro-carbons found along willi them; but, significantly enough, it includes several of those substances classed as inorganic, which enter into organized structures. Thus silica, which is a component of many plants, and constitutes the spicules of sponges as well as the shells of many foraminifora and infusoria, has a colloid, as well as a crystalloid, condition. A solution of hydrated silicic acid, passes in the course of a few days into a solid jelly that is no longer soluble in water; and it may be suddenly thus coagulated by a minute portion of an alkaline carbonate, as well as by gelatine, alumina, and peroxide of iron. This lastnamed substance, too — peroxide of iron — which is an ingredient in the blood of mammals and composes the shells of certain protozoa, has a colloid condition. '' Water containing about one per cent, of hydrated peroxide of iron in solution, has the dark red colour of venous blood." * * * a ^}^g red solution is coagulated in the cold by traces of sulphuric acid, alkalies, alkaline carbonates, sulphates, and neutral salts' in general." * * * *' The coagulum is a deep red-coloured jelly, resembling the clot of blood but more transparent.
Indeed, the coagulum of this colloid is highly suggestive of that of blood, from the feeble agencies which suffice to effect the change in question, as well as from the appearance of the product." The jelly thus formed soon becomes, like the last, insoluble in water. Lime also, which is so important a mineral element in living bodies, animal and vegetal, enters into a compound belonging to this class. *' The well-known solution of lime in sugar, forms a solid coagulum when heated. It is probably, at a high temperature, entirely colloidal."
Generalizing some of the facts which he gives. Professor Graham says — " The equivalent of a colloid appears to be always high, although the ratio between the elements of the substance may be simple. Gummic acid, for instance, may be represented by C^^ H^^ 0^^; but, judging from the small proportions of lime and potash which suffice to neutralize this acid, the true numbers of its formula must be several times greater. It is difficult to avoid associating the inertness of colloids with their high equivalents, particularly where the high number appears to be attained by the repetition of a small number. The inquiry suggests itself whether the colloid molecule may not be constituted by the grouping together of a number of smaller cr3^stalloid molecules, and whether the basis of colloidality may not really be this composite character of the molecule.''
§ 7. A further contrast between colloids and crj'stalloids, is equally significant in its relations to vital phenomena. Professor Graham points out that the marked differences in volatility displaj^ed by different bodies, are paralleled by differences in the rates of diffusion of different bodies through liquids. As alcohol and ether at ordinary temperatures, and various other substances at higher temperatures, diffuse themselves in a gaseous form through the air; so, a substance in aqueous solution, when placed in contact with a mass of water (in such way as to avoid mixture by circulating currents) diffuses itself through this mass of water. And just as there are various degrees of rapidity in evaporation, so there are various degrees of rapidity in diffusion: ** the range also in the degree of diffusive mobility exhibited by different substances appears to be as wide as the scale of vapour- tensions." This parallelism is what might have been looked for; since the tendency to assume a gaseous state, and the tendency to spread in solution through a liquid, are both consequences of molecular mobility. It also turns out, as was to be expected, that diffusibility, like volatilit}^, has, other things equal, a relation to atomic weight — (other things equal, we must say, because molecular mobility must, as pointed out in § 5, be affected by other properties of atoms, besides their inertia).
Thus the substance most rapidly diffused of any on which Professor Graham experimented, was hydro-chloric acid — a compound which is of low atomic weight, is gaseous save under a pressure of forty atmospheres, and ordinarily exists as a liquid, only in combination with water. Again, '' hydrate of potash may be said to possess double the velocity of diffusion of sulphate of potash, and sulphate of potash again double the velocity of sugar, alcohol, and sulphate of magnesia," — differences which have a general correspondence with differences in the massiveness of the atoms.
But the fact of chief interest to us here, is that the relatively small-atomed crystalloids have immensely greater diffusive power than the relatively large-atomed colloids. Among the crystalloids themselves, there are marked differences of diffusibility; and among the colloids themselves, there are parallel differences, though less marked ones. But these differences are small compared with that between the diffusibility of the crystalloids as a class, and the diffusibility of the colloids as a class. Hydro-chloric acid is seven times as diffusible as sulphate of magnesia; but it is fifty times as diffusible as albumen, and a hundred times as diffusible as caramel.
These differences of diffusibility manifest themselves with nearly equal distinctness, when a permeable septum is placed between the solution and the water. And the result is, that when a solution contains substances of different diffusibilities, the process of dialysis, as Professor Graham calls it, becomes a means of separating the mixed substances: especially when such mixed substances are partly crystalloids and partly colloids. The bearing of this fact on organic processes will be obvious. Still more obvious will its bearing be, on joining it with the remarkable fact, that while crystalloids can diffuse themselves through colloids nearly as rapidly as through water, colloids can scarcely diffuse themselves at all through other colloids. From a mass of jelly containing salt, into an adjoining mass of jelly containing no salt, the salt spread more in eight days than it spread through water in seven days; while the spread of '' caramel through the jelly appeared scarcely to have begun after eight days had elaiDsed." So that we must regard the colloidal compounds of which organisms are built, as having by their physical nature, the ability to separate colloids from crystalloids, and to let the crystalloids pass through them with scarcely any resistance.
One other result of these researches on the relative diflfusibilities of different substances, has a meaning for us. Professor Graham finds, that not only does there take place by dialysis, a separation of mixed substances which are unlike in their molecular mobilities; but also that comhined substances between which the afiinity is feeble, will separate on the dialyzer, if their molecular mobilities are strongly contrasted. Speaking of the hydro-chlorate of peroxide of iron, he says, " such a compound possesses an element of instability in the extremely unequal diffusibility of its constituents; " and he points out that when dialyzed, the hj^dro- chloric acid gradually difi'uses away, leaving the colloidal peroxide of iron behind. Similarly, he remarks of the peracetate of iron, that it " may be made a source of soluble peroxide, as the salt referred to is itself decomposed to a great extent by diffusion on the dialyzer." Kow this tendency to separate displayed by substances that differ widely in their molecular mobilities, though usually so far antagonized by their affinities as not to produce spontaneous decomposition, must, in all cases, induce a certain readiness to change which would not else exist. The unequal mobilities of the combined atoms, must give disturbing forces a greater power to work transformations than they would otherwise have. Hence the probable significance of a fact named at the outset, that while three of the chief organic elements have the greatest atomic mcbilities of any elements known, the fourth, carbon, has the least atomic mobility of known elements. Though, in its simple compounds, the affinities of carbon for the rest are strong enough to prevent the effects of this great difference from clearly showing themselves; yet there seems reason to thiukj that in those com- OKGANIC MAITKR. 21 plcx compounds composing organic bodies — compounds in which there are various cross afRnitics leading to a state of chemical tension — this extreme difference in the molecular mobilities must be an important aid to molecular re-arrangements. In short, we are here led by concrete evidence to the conclusion which we before drew from first principles, that this great unlikeness among the combined units must facilitate differentiations.
§ 8. A portion of organic matter In a state to exhibit those phenomena which the biologist deals with, is, however, something far more complex than the separate organic matters we have been studying; since a portion of organic matter in its integrity, contains several of these.
In the first place, no one of those colloids which make up the mass of a living body, appears capable of carrying on vital changes by itself: it is always associated with other colloids. A portion of animal-tissue, however minute, almost always contains more than one form of protein-substance: different chemical modifications of albumen and gelatine are present together, as well as, probably, a soluble and insoluble modification of each; and there is usually more or less of fatty matter. In a single vegetal cell, the minute quantity of nitrogenous colloid present, is imbedded In colloids of the non-nitrogenous class. The microscope makes it at once manifest, that even the smallest and simplest organic forms are not absolutely homogeneous.
Further, we have tol contemplate organic tissue, formed of mingled colloids in both soluble and insoluble states, as permeated throughout by crystalloids. Some of these crystalloids, as oxygen,* water, and perhaps certain salts, are agents of decomposition; some, as the saccharine and fatty * It will perhaps seem strange to class oxygen as a crystalloid. But inasmuch as the crystalloids are distinguished from the colloids by their atomic simplicity, and inasmuch as sundry gases are reducible to a crystalline state, we are justified Iq so classing^ it.
matters, are probably materials for decomposition; and some, as carbonic acid, water, urea, kreatine, and kreatinine, are products of decomposition. Into the mass of mingled colloids, mostly insoluble and where soluble of very low molecular mobility or diffusive power, we have constantly passing, crystalloids of high molecular mobility or diffusive power, that are capable of decomposing these complex colloids; and from these complex colloids, so decomposed, there result other crystalloids (the two chief ones extremely simple and mobile, and the rest comparatively so) which diffuse away as rapidly as they are formed.
And now we may clearly see the necessity for that peculiar composition which we find in organic matter. On the one hand, were it not for the extreme molecular mobility possessed by three of its chief elements out of the four; and were it not for the consequently high molecular mobility of their simpler compounds; there could not be this quick escape of the waste products of organic action; and there could not be that continuously active change of matter which vitality implies. On the other hand, were it not for the union of these extremely mobile elements into immensely complex compounds, having relatively vast atoms that are made comparatively immobile by their inertia, there could not result that mechanical fixity which prevents the components of living tissue from diffusing away along with the effete matters produced by the decomposition of tissue.
§ 9. Thus in the substances of which organisms are composed, the conditions necessary to that re- distribution of Matter and Motion which constitutes Evolution, are fulfilled in a far higher degree than at first appears.
The mutual affinities of the chief organic elements are not active within the limits of those temperatures at which organic actions take place; and one of these elements is especially characterized by its chemical indifference. The compounds formed by these elements in ascending grades of complexity, become progressively less stable. And those most complex compounds into which all these four elements enter, together with small proportions of two other elements that very readily oxidize, have an instability so great that decomposition ensues under ordinary atmospheric conditions.
Among these elements out of which living bodies are built, there is an unusual tendency to unite in multiples; and so to form groups of products which have the same chemical components, but, being different in their modes of aggregation, possess different properties. This prevalence among them of isomerism and polymerism, shows, in another way, the special fitness of organic substances for undergoing re-distributions.
In those most complex compounds that are instrumental to vital actions, there exists a kind and degree of molecular mobility which constitutes the plastic quality fitting them for organization. Instead of the extreme molecular mobility possessed by three out of the four organic elements in their separate states — instead of the diminished, but still great, molecular mobility possessed by their simpler combinations, the gaseous and liquid characters of which unfit them for showing to any extent the process of Evolution — instead of the properties of their less simple combinations, which, when not made unduly mobile by heat, assume the unduly rigid form of crystals; we have in these colloids, of which organisms are mainly composed, just the required compromise between fluidity and solidity. They cannot be reduced to the unduly mobile conditions of liquid and gas; and yet they do not assume the unduly fixed condition usually characterizing solids. The absence of power to unite together in polar arrangement, leaves their atoms with a certain freedom of relative movement which makes them sensitive to small forces, and produces plasticity in the aggregates composed of them.
While the relatively great inertia of these large and complex organic atoms, renders them comparatively incapable of being set in motion by the ethereal undulations, and so reduced to less coherent forms of aggregation; there is reason to think that this same inertia facilitates changes of arrangement among their constituent atoms; since, in proportion as an incident force impresses but little motion on a mass, it is the better able to impress motion on the parts of the mass in relation to each other. And it is further probable that the extreme contrasts in molecular mobilities among the components of these highly complex atoms, aid in producing modifiability of arrangement among them.
Lastly, the great difference in diifusibility between colloids and crystalloids, makes possible in the tissues of organisms, a specially rapid re-distribution of matter and motion; both because colloids, being easily permeable by crystalloids, can be chemically acted on throughout their whole mass, instead of only on thsir surfaces; and because the products of decomposition, being also crystalloids, can escape as fast as they are produced, leaving room for further like transformations. So that while the composite atoms of which organic tissues are built up, possess that low molecular mobility fitting them for plastic purposes, it results from the extreme molecular mobilities of their ultimate constituents, that the waste products of vital activity escape as fast as they are formed.
To all which add, that the state of warmth, or increased molecular vibration, in which all the higher organisms are kept, increases these various facilities for re-distribution: not only as aiding chemical changes, but as accelerating the diffusion of crystalloid substances.
CHAPTER II.
§ 10. To some extent, the parts of every body are changed in their arrangement by any incident mechanical force. But in organic bodies, the changes of arrangement produced by mechanical forces are usually conspicuous. It is a distinctive mark of colloids, that they yield with great readiness to pressures and tensions; and that they yet recover, more or less completely, their original shapes, when the pressures or tensions cease. It is clear that without this pliability and elasticity, most organic actions would be impossible. Not only temporary but permanent alterations of form are facilitated by this colloid character of organic matter. Continued pressure on living tissue, by modifying the processes going on in it, (perhaps retarding the absorption of new material to replace the old that has decomposed and diffused away,) gradually diminishes and finally destroys its power of resuming the outline it had at first. Thus the matter of which organisms are built up, is modifiable by arrested momentum or by continuous strain, in a far greater degree than is ordinary matter.
§ 11. Sensitiveness to certain forces that are quasimechanical, if not mechanical in the usual sense, is seen in two closely- related peculiarities displayed by organic matter as well as other matter that assumes the same state of molecular aoiffreofation.
Colloids take up by a "power that has been called " capillary affinity," a large quantity of water: undergoing at the same time great increase of bulk with change of form. Conversely, with like readiness, they give up this water by evaporation: resuming more or less completely their original states. Whether resulting from capillarity, or from the relatively great diffusibility of water, or from both; these changes are to be here noted as showing another mode in which the arrangement of parts in organic bodies, is affected by mechanical forces.
In what is called osmose, we have a further mode of allied kind. When on opposite sides of a permeable septum, and especially a septum of colloidal substance, are placed miscible solutions of different densities, a double transfer takes place: a large quantity of the less dense solution finds its way through the septum into the more dense solution; and a small quantity of the more dense finds its way into the less dense — one result being a considerable increase in the bulk of the more dense at the expense of the less dense. This process, which appears to depend on several conditions, is not yet fully understood. But be the explanation what it may, the process is one that tends continually to work alterations in organic bodies. Through the surfaces of plants and animals, transfers of this kind are ever taking place. Yery many of the conspicuous changes of form undergone by organic germs, are due mainly to the permeation of their limiting membranes by the surrounding liquids.
It should be added that besides the direct alterations which the imbibition and transmission of water and watery solutions by colloids produce in organic matter, they produce indirect alterations. Being instrumental in conveying into the tissues the agents of chemical change, and conveying out of them the products of chemical change, they aid in carrying on other re- distributions.
§ 12. As elsewhere shown (Fird Principles, § 103) Heat, or a raised state of molecular vibration, enables incident forces more easily to produce changes of molecular arrangement in organic matter. But besides this, it conduces to certain vital changes in so direct a way as to become their chief cause.
The power of the organic colloids to imbibe water, and to bring along with it into their substance the materials which work transformations, would not be continuously operative if the water imbibed were to remain. It is because it escapes, and is replaced by more containing more materials, that the succession of changes is maintained. Among the higher animals and higher plants its escape is facilitated by evaporation. And the rate of evaporation is, other things equal, determined by heat. Though the current of sap in a tree is mainly caused by some action, probably osmotic, that is at work in the roots; yet the loss of water from the surfaces of the leaves, and the consequent absorption of more sap into the leaves by capillary attraction, must largely aid the circulation. The drooping of a plant when exposed to the sunshine while the earth round its roots is dry, shows us how evaporation empties the sap-vessels; and the quickness with which a withered slip revives on being placed in water, shows us the part which capillary action plays.- In so far then, as the evaporation from a plant's surface helps to produce currents of sap through the plant, we must regard the heat which produces this evaporation as a part-cause of those re-distributions of matter which these currents effect. In terrestrial animals, heat similarly aids the changes that are going on. The exhalation of vapour from the lungs and the surface of the skin, forming the chief escape of the water that is swallowed, conduces to the maintenance of those currents through the tissues, without which the functions would cease. For though the vascular system distributes nutritive fluids in ramified channels through the body; yet the absorption of these fluids into tissues, partly depends on the escape of fluids which the tissues already contain. Hence, to the extent that such escape is facilitated by evaporation, and this evaporation facilitated by heat, heat becomes an agent of re-distribution in the animal organism.
§ 13. Light, which is now known to modify many inorganic compounds — which works those chemical changes utilized in photography, causes the combinations of certain gases, alters the molecular arrangements of many crystals, and leaves traces of its action even on substances that are extremely stable, — may be expected to produce marked effects on substances so complex and unstable as those which make up organic bodies. It does produce such marked effects; and some of them are among the most important that oro^anic matter nndersfoes.
The molecular changes wrought by light in animals, are but of secondary moment. There is the darkening of the skin that follows exposure to the sun's rays. There are those alterations in the retina which cause in us sensations of colours. And on certain eyeless creatures that are semitransparent, tbe light permeating their substance works some effect evinced by movement. But speaking generally, the opacity of animals limits the action of light to their surfaces; and so renders its direct physiological influence but small.* On plants, however, the solar rays that produce in us the impression of yellow, are the immediate agents of those molecular changes through which are hourly accumulated the materials for further growth. Experiments have shown that when the sun shines on living leaves, they begin to exhale oxygen and to accumulate carbon and hydrogen — results which are traced to the decomposition by the solar rays, of the carbonic acid and water absorbed. It is now an accepted conclusion that, by the help of certain * The increase of respiration found to result from the presence. of light, is probably an indirect, effect. It is most likely due to the reception of more vivid impressions through the eyes, and to the consequent nervous stimulation.
'illE ACTIONS OF FOllCKS ON ORGANIC MATTER. 20 classes of the ethereal undulations penetrating their leaves, plants are enabled to separate from the associated oxygen, those two elements of which their tissues are chiefly built This transformation of ethereal undulations into certain molecular re-arrangements of an unstable kind, on the overthrow of which the stored-up forces are liberated in new forms, is a process that underlies all organic phenomena. It will therefore be well, if we pause a moment to consider whether any proximate interpretation of it is possible. Certain recent researches in molecular physics, give us some clue to its nature.
The elements of the problem are these: — The atoms of several ponderable matters exist in combination: those that are combined having strong affinities, but having also affinities less strong for some of the surrounding atoms that arc otherwise combined. The atoms thus united, and thus mixed among others with which they are capable of uniting, are exposed to the undulations of a medium that is relatively so rare as to seem imponderable. These undulations are of numerous kinds: they differ greatly in their lengths, or in the frequency with which they recur at any given point. And under the influence of undulations of a certain frequency, some of these atoms are transferred from atoms for which they have a stronger aflinity, to atoms for which they have a weaker affinity. That is to say, particular orders of waves of a relatively imponderable matter, remove particular atoms of ponderable matter from their attachments, and carry them within reach of other attachments. Now the discoveries of Bunsen and Kirchoif respecting the absorption of particular luminiferous undulations by the vapours of particular substances, joined with Prof. Tyndall's discoveries respecting the absorption of heat by gases, show very clearly that the atoms of each substance have a rate of vibration in harmony with ethereal waves of a certain length, or rapidity of recurrence. Every special kind of atom can be made to oscillate by a special order of ethereal waves, which are absorbed in producing its oscillations; and can by its oscillations generate this same order of ethereal waves. Whence it appears that immense as is the difference in density between ether and ponderable matter, the waves of the one can set the atoms of the other in motion, when the successive impacts of the waves are so timed as to correspond with the oscillations of the atoms. The effects of the waves are, in such case, cumulative; and each atom gradually acquires a momentum made up of countless infinitesimal momenta. Note further, that unless the members of a chemically-compound atom are so bound up as to be incapable of any relative movements (a supposition at variance with the conceptions of modern science) we must conceive them as severally able to vibrate in unison or harmony with those same classes of ethereal waves that affect them in their uncombined states. While the compound atom as a whole, will have some new rate of oscillation determined by its attributes as a whole; its components will retain their original rates of oscillation, subject only to modifications by mutual influence. Such being the circumstances of the case, we may partially understand how the sun's raj^s can effect chemical decompositions. If the members of a binary atom stand so related to the undulations falling on them, that one is thrown into a state of increased oscillation and the other not; it is manifest that there must arise a tendency towards the dislocation of the two — a tendency which may or may not take effect, according to the weakness or strength of their union, and according to the presence or absence of collateral affinities. This inference is in harmony with several significant facts. Dr Draper remarks that " among metallic substances (compounds) those first detected to be changed by light, such as silver, gold, mercury, lead, have all high atomic weights; and such as sodium and potassium, the atomic weights of which are low, appeared to be less changeable.'' As here interpreted, the fact specified amounts to this; that the compounds most readily decomposed by light, are those in wliich there is a marked contrast between the atomic weights of the constituents, and probably therefore a marked contrast between the rapidities of their vibrations. The circumstance, too, that different chemical compounds are decomposed or modified in different parts of the spectrum, implies that there is a relation between special orders of undulations and special orders of composite atoms — doubtless a correspondence between the rates of these undulations and the rates of oscillation which some of the components of such atoms will assume. Strong confirmation of this view may