"Life is, therefore, quite inseparable from chemical reactions, and on the whole what we call life is nothing else but a complex of innumerable chemical reactions in the living substance which we call protoplasm."[1]
The essential quality, therefore, of life is continual change, but not mere change in general. It is that special process of the circulation of matter which we call metabolism, and this circulation is always connected with a particular chemical substance--protoplasm.
In this substance five elements are always present and predominant--carbon, oxygen, nitrogen, hydrogen, and sulphur. The compounds which these five elements form with each other are most complex and varied, and they also admit to combination--but in smaller proportions--some of the other elements, of which phosphorus, potassium, calcium, magnesium, and iron are the most important.
For protoplasm--using the term in the most general sense--is a chemical substance, not a mere mixture of a number of chemical elements, nor a mere mechanical structure. "However differently the various plasma substances behave in detail, they always exhibit the same general composition as the other albuminoids out of the five 'organo-genetic elements'--namely in point of weight, 51-54% carbon, 21-23% oxygen, 15-17% nitrogen, 6-7% hydrogen, and 1-2% sulphur."[2]
Haeckel, the writer just quoted, describes the plasm, the universal basis of all the vital phenomena, in the following terms: "In every case where we have with great difficulty succeeded in examining the plasm as far as possible and separating it from the plasma-products, it has the appearance of a colourless, viscous substance, the chief physical property of which is its peculiar thickness and consistency. The physicist distinguishes three conditions of inorganic matter--solid, fluid, and gaseous. Active living protoplasm cannot be strictly described as either fluid or solid in the physical sense. It presents an intermediate stage between the two which is best described as viscous; it is best compared to a cold jelly, or solution of glue. Just as we find the latter substance in all stages between the solid and the fluid, so we find in the case of protoplasm. The cause of this softness is the quantity of water contained in the living matter, which generally amounts to a half of its volume and weight. The water is distributed between the plasma molecules or the ultimate particles of living matter in much the same way as it is in the crystals of salts, but with the important difference that it is very variable in quantity in the plasm. On this depends the capacity for the absorption or imbibition in the plasm, and the mobility of its molecules, which is very important for the performance of the vital actions. However, this capacity of absorption has definite limits in each variety of plasm; living plasm is not soluble in water, but absolutely resists the penetration of any water beyond this limit."[3] And Czapek further tells us that "the most striking feature of cell life is the fact that an enormous number of chemical reactions take place within the narrowest space. Most plant cells do not exceed 0·1 to 0·5 millimetres in diameter. Their greatest volume therefore can only be an eighth of a cubic millimetre. Nevertheless, in this minute space we notice in every stage of cell life a considerable number of chemical reactions which are carried on contemporaneously, without one disturbing the other in the slightest degree."[4]