THE SOURCES OF ENERGY 59 



genated " vegetable oils, plant oils such as olive, cotton-seed, 

 palm, etc.). 



Proteids are innumerable, and the flesh of almost every species 

 of animal contains distinct kinds. They are the most complex 

 of all chemical substances, and it is only during the last quarter 

 of a century that they have been successfully investigated 

 (mainly by Fischer and the German chemists of his school). All 

 of them contain the chemical elements carbon, hydrogen, oxygen, 

 and nitrogen, united together in a very complex way. They are 

 now known to be built up of peculiar substances called amino- 

 acids, and one of the simpler examples of the latter has the 

 following formula and name: 



^!^'*>CH— CH— CH(NH,)— COOH. 



Leucine or a-amino-iso-caproic acid. 



Very many of these amino-acids are known, but many more 

 still have yet to be investigated. Now imagine a number of 

 these united together in this way : 



R' R' R'" 



I I I 



NH,— CH— CO ^NH— CH— CO NH— CH— COOH. 



(1) (2) (3) 



Here we have a chain of members, each of which has the form : 



R 



I 

 — NH— CH— CO— ; 



the N, H, C, and each represent an atom of nitrogen, 

 hydrogen, carbon, and oxygen respectively; and R represents a 

 complex ""radicle " or group of atoms such as (in a simple case): 



CH3— CHa— CH— . 



In the example we have given the chain consists of only three 

 members, but there may be very many more. It may be bent 

 round to form a ring, and the ring may be joined to other rings, 

 and there may be " side chains " attached in various ways. Now 

 compare all this with the chemical formula of urea (which is the 

 characteristic substance present in the urine): 



„p NH2 



