CHEMICAL COMPOSITION OF PROTOPLASM 23 



different kinds of proteids are known to us and have received 

 special names ; such are the albumen which occurs in the white 

 of an egg, the casein which is met with in cheese, the legumin 

 which is characteristic of peas and beans, the gliadin and 

 glutinin of flour, and so forth. 



These proteids are, for the most part at any rate, colloid 

 substances, that is to say they are more or less gelatinous and 

 incapable of diffusing through organic membranes. This may 

 be accounted for if we assume that the highly complex molecules 

 of which they are composed are too large to pass through the 

 very minute pores which occur in such membranes and which 

 readily allow of the passage of the molecules of simpler, 

 crystalloid substances. The colloid nature of the proteid con- 

 stituents of protoplasm plays a very important part in determining 

 its properties and behaviour. Crystalloid mineral salts and other 

 diffusible substances in a state of solution can pass through a 

 cell-wall or membrane by osmosis, and thus the living proto- 

 plasm receives fresh supplies of nutriment, but the colloid 

 proteids are as a rule formed inside the cell and cannot usually 

 pass out again until they have undergone some chemical change 

 whereby they are rendered diffusible. 



The mineral salts which we find in the protoplasm, usually- in 

 a state of solution, are of very various kinds, compounds of 

 sodium, potassium, calcium and other elements with various 

 inorganic" and organic acids, such as sulphuric, hydrochloric, 

 malic and citric acids. 



Finally, water must always be present in living protoplasm and 

 usually forms a very large percentage of the whole mass. 



Whatever view we may take with regard to the question of 

 vitalism, there can be no doubt that the most distinctive property 

 of living protoplasm is its power of controlling chemical and 

 physical processes so as to make them yield results different from 

 those which would be obtained if we were dealing with not-living 

 matter. The various processes upon which depend the functions 

 of movement, nutrition, respiration and excretion all appear to be 

 controlled in this manner, but the general principle is perhaps 

 most beautifully illustrated in the case of many of the lower 

 animals and plants, in which the protoplasm secretes a protective 

 or supporting skeleton of some mineral substance, such as silica, 

 or carbonate of lime. Silica, for example, in the inorganic world, 

 occurs abundantly in a state of solution in water, from which it 



