LIVING SUBSTANCE 9 



weeds and in the thyroid gland of certain animals. 

 Zinc and manganese seem to be normal constituents 

 of the tissues of some mollusks. In other words, 

 protoplasm is not a definite chemical substance or 

 compound, like quartz or salt or starch, but is some- 

 times one thing, sometimes another. Rather, it N is 

 a mixture of various things, all of them, however, 

 of an infinite complexity of mutual relations, 

 " a mixture, but certainly no jumble." The word 

 "protoplasm," then, is a sort of group name covering 

 a multitude of different sorts of such chemical mix- 

 tures, as many as there are different manifestations 

 of life phenomena. 



It has been just said that the bulk- of all kinds of 

 protoplasm is made up of carbon, hydrogen, oxygen, 

 and nitrogen. These exist in elaborate combina- 

 tions, of which the carbon atom seems, as a rule, 

 to form the heart or foundation. The older "or- 

 ganic chemistry," or the chemistry of organisms and 

 their products, has become the " chemistry of the 

 carbon compounds." 



An exception must be made to the statement 

 that the combinations of the four elements cited 

 are always complex. One of the simplest of all 

 compounds, water (H 2 O), is a necessary constit- 

 uent of living matter. The percentage of water in 

 all protoplasm is high. Muscles are three fourths 

 water, even bones, nearly one fourth, and in the 

 jellyfishes of the open sea, that which is not water 

 is but one per cent or less of the total bulk. With 

 these facts* in mind, one is inclined to think of living 



