298 EFFECT OF CHEMICAL AGENTS [Ch. XI 



greater part of the entire dry matter. The relations of the 

 remaining elements are largely obscure. Some of them form 

 inorganic acids in the body, such as hydrochloric and sulphuric 

 acids. Others form inorganic compounds deposited in the body 

 as supporting or protective substances, such as the calcic phos- 

 phate and calcic carbonate of bone and spicules and the silicic 

 oxide of plants. But there can be little doubt that a large and 

 highly important part of these elements is built up into organic 

 molecules and in this position plays a weighty and varied part 

 in metabolism and growth. As examples of the way in which 

 the metals and metalloids occur in the organic molecule I may 

 cite a few cases in which the molecular structure has been deter- 

 mined more or less satisfactorily. Thus we have sulphur in albu- 

 men, C^gHjigNjgSOjg ; iron in hematin, Cg^HgglSr^FeOg ; phos- 

 phorus in lecithin, C42HgjNPOg, and nuclein, C29H4gNgPg022) 

 magnesium in chlorophyllan, and various halogens in the 

 urates of sodium, calcium, and lithium. In discussing as we 

 shall immediately the importance of each of these elements for 

 the formation of the body we shall find additional facts con- 

 cerning the importance of the metallic atoms in the organic 

 molecules. There is good reason for believing that the pecul- 

 iar properties of haemoglobin depend upon its iron and that 

 the characteristic properties of nuclein depend largely upon its 

 phosphorus and (as the later investigations indicate) its iron 

 also. As our knowledge develops, the importance to many 

 organic molcules in the living body of metallic or metalloid 

 elements becomes clearer; the "ash" of the body is not some- 

 thing accidental, secondary, or superfluous, but is an essential 

 part of the organism. 



We must now consider the source of the various elements 

 necessary to nutrition. Botanists early determined that C, H, 

 and O enter the organism through the carbonic acid and water 

 which green plants absorb. Concerning the source of nitrogen 

 there has been less certainty ; it is generally believed to come 

 from nitrates in the soil, but that matter will be considered 

 later in more detail. Another source of the more characteris- 

 tically organic elements, C, H, N, and O, is, without doubt, 

 organic compounds of various sorts. Although it has long 

 been known that insectivorous plants absorb the organic mat- 



