We are told, for instance, that the powerful force of surface-tension, or capillarity, begins to act within a range of about 1ã₤ãã₤500,000 of an inch, or say 0ôñ05 ôç. A soap-film, or a film of oil upon water, may be attenuated to far less magnitudes than this; the black spots upon a soap-bubble are known, by various concordant methods of measurement, to be only about 6ã₤ûã₤10ÿ£¢ã7 cm., or about ôñ006 ôç thick, and Lord Rayleigh and M. Devaux49 have obtained films of oil of ôñ002 ôç, or even ôñ001 ôç in thickness.
But while it is possible for a fluid film to exist in these almost molecular dimensions, it is certain that, long before we reach them, there must arise new conditions of which we have little knowledge and which it is not easy even to imagine.
It would seem that, in an organism of ôñ1 ôç in diameter, or even rather more, there can be no essential distinction between the interior and the surface layers. No hollow vesicle, I take it, can exist of these dimensions, or at least, if it be possible for it to do so, the contained gas or fluid must be under pressures of a formidable kind50, and of which we have no knowledge or experience. Nor, I imagine, can there be any real complexity, or heterogeneity, of its fluid or semi-fluid contents; there can be no vacuoles within such a cell, nor any layers defined within its fluid substance, for something of the nature of a boundary-film is the necessary condition of the existence of such layers. Moreover, the whole organism, provided that it be fluid or semi-fluid, can only be spherical in form. What, then, can we attribute, in the way of properties, to an organism of a size as small as, or smaller than, say ôñ05 ôç? It must, in all probability, be a homogeneous, structureless sphere, composed of a very small number of albuminoid or other molecules. Its vital properties and functions must be extraordinarily limited; its specific outward characters, even if we could see it, must be nil; and its specific properties must be little more than those of an ion-laden corpuscle, enabling it to perform {44} this or that chemical reaction, or to produce this or that pathogenic effect. Even among inorganic, non-living bodies, there must be a certain grade of minuteness at which the ordinary properties become modified. For instance, while under ordinary circumstances cryôÙstalôÙliôÙsaôÙtion starts in a solution about a minute solid fragment or crystal of the salt, Ostwald has shewn that we may have particles so minute that they fail to serve as a nucleus for cryôÙstalôÙliôÙsaôÙtion,ãwhich is as much as to say that they are too minute to have the form and properties of a ãcrystalã; and again, in his thin oil-films, Lord Rayleigh has noted the striking change of physical properties which ensues when the film becomes attenuated to something less than one close-packed layer of molecules51.
Thus, as Clerk Maxwell put it, ãmolecular science sets us face to face with physiological theories. It forbids the physiologist from imagining that structural details of infinitely small dimensions [such as Leibniz assumed, one within another, ad infinitum] can furnish an explanation of the infinite variety which exists in the properties and functions of the most minute organisms.ã And for this reason he reprobates, with not undue severity, those advocates of pangenesis and similar theories of heredity, who would place ãa whole world of wonders within a body so small and so devoid of visible structure as a germ.ã But indeed it scarcely needed Maxwellãs criticism to shew forth the immense physical difficulties of Darwinãs theory of Pangenesis: which, after all, is as old as Democritus, and is no other than that Promethean particulam undique desectam of which we have read, and at which we have smiled, in our Horace.