45_a_. Two points must be noted respecting organic substances: they are mostly combinations of _higher multiples_ of the elements; and their combinations are not _definite_ in quantity. Albumen, for example, has (according to one of the many formulas which have been given) an elementary composition of 216 atoms of Carbon, 169 of Hydrogen, 27 of Nitrogen, 3 of Sulphur, and 68 of Oxygen; whereas in its final state, in which it quits the organism as Urea, it is composed of 2 atoms of Carbon, 4 of Hydrogen, 2 of Nitrogen, and 2 of Oxygen, all the Sulphur having disappeared in other combinations. In like manner in the organism Stearin falls from C_{114}, H_{110}, O_{12}, to Oxalic Acid, which is C_{4}, H_{2}, O_{8}. It is obvious that the necessary modifiability of organic substance is due to this multiplicity of its elementary parts and the variety of its molecular structure. 45_b_. Nor is the indefiniteness of the quantitative composition less important, though seldom adequately appreciated, or even suspected. Robin and Verdeil[11] are the only writers I can remember who have distinctly brought the fact into prominence. That all inorganic substances are definite in composition, every one knows. Quicklime, for example, may be got from marble, limestone, oyster-shells, or chalk; but however produced, it always contains exactly 250 ounces of calcium to 100 ounces of oxygen; just as water is always OH_{2}. Not so the pre-eminently vital substances, those which are coagulable and not crystallizable: no precise formula will express one of these; for the same specific substance is found to vary from time to time, and elementary analyses do not give uniform results. Thus, if after causing an acid to combine with one of these substances, we remove the acid, we are not certain of finding the substance as it was before--as we are, for example, after urea is combined with nitric acid and then decomposed. The same want of definiteness is of course even more apparent in the _combinations_ of these proximate principles into organized substance. Protoplasm differs greatly in different places. Epithelial cells differ. Muscular and nervous fibres are never absolutely the same in different regions. A striped and unstriped muscular fibre, the muscular fibre of a sphincter or of a limb, a nerve-fibre in a centre, in a trunk, or in a gland, will present variations of composition. The elastic fibres of the ligaments are larger in the horse than in man; and in other animals they are smaller. These differences are sometimes due to the constituents, and sometimes to the arrangement of the constituents; the conversion of Albumen into Fibrine without elementary loss or addition, is a good example of the latter. That the tissues of one man are not absolutely the same as the tissues of another, in the sense in which it is true to say that the chalk of one hill is the same as that of another, or as gold in Australia is the same as gold in Mexico, is apparent in their very