That there is a close likeness between all these bodies is obvious from the fact that when any of them is strongly heated, or allowed to putrefy, it gives off the same sort of disagreeable smell; and careful chemical analysis has shown that they are, in fact, all composed of the elements Carbon, Hydrogen, Oxygen, and Nitrogen, combined in very nearly the same proportions. Indeed, charcoal, which is impure carbon, might be obtained by strongly heating either a handful of corn, or a piece of fowl’s flesh, in a vessel from which the air is excluded so as to keep the corn or the flesh from burning. And if the vessel were a still, so that the products of this destructive distillation, as it is called, could be condensed and collected, we should find water and ammonia, in some shape or other, in the receiver. Now ammonia is a compound of the elementary bodies, nitrogen and hydrogen; therefore (§ 50) both nitrogen and hydrogen must have been contained in the bodies from which it is derived.
It is certain, then, that very similar nitrogenous compounds form a large part of the bodies of both the wheat plant and the fowl, and these bodies are called proteids.
59. Proteid Substances are met with in Nature only in Animals and Plants; and Animals and Plants always contain Proteids.
It is a very remarkable fact that not only are such substances as albumin, gluten, fibrin and syntonin, known exclusively as products of animal and vegetable bodies; but that every animal and every plant, at all periods of its existence, contains one or other of them, though, in other respects, the composition of living bodies may vary indefinitely. Thus, some plants contain neither starch nor cellulose, while these substances are found in some animals; while many animals contain no horny matter and no gelatin-yielding substance. So that the matter which appears to be the essential foundation of both the animal and the plant is the proteid united with water; though it is probable that, in all animals and plants, these are associated with more or less fatty and amyloid (or starchy and saccharine) substances, and with very small quantities of certain mineral bodies, of which the most important appear to be phosphorus, iron, lime, and potash.
Thus there is a substance composed of water + proteids + fat + amyloids + mineral matters which is found in all animals and plants; and, when these are alive, this substance is termed protoplasm.
60. What is meant by the word Living?
The wheat plant in the field is said to be a living thing; the fowl running about the farm-yard is also said to be a living thing. If the plant is plucked up, and if the fowl is knocked on the head, they soon die and become dead things. Both the fowl and the wheat plant, as we have seen, are composed of the same elements as those which enter into the composition of mineral matter, though united into compounds which do not exist in the mineral world. Why then do we distinguish this matter when it takes the shape of a wheat plant, or a fowl, as living matter?
61. The Living Plant increases in size, by adding to the Substances which compose its Body, like Substances; these, however, are not derived from without, but are manufactured within the Body of the Plant from simpler Materials.
In the spring, a wheat-field is covered with small green plants. These grow taller and taller until they attain many times the size which they had when they first appeared; and they produce the heads of flowers which eventually change into ears of corn.
In so far as this is a process of growth, accompanied by the assumption of a definite form, it might be compared with the growth of a crystal of salt in brine: but, on closer examination, it turns out to be something very different. For the crystal of salt grows by taking to itself the salt contained in the brine, which is added to its exterior; whereas the plant grows by addition to its interior: and there is not a trace of the characteristic compounds of the plant’s body, albumin, gluten, starch, or cellulose, or fat, in the soil, or in the water, or in the air.