EVOLUTION OF PROTEIDS 73 



posed of two molecules of urea and one of acrylic 

 acid united with loss of three H atoms and a mole- 

 cule of water. Xanthin by connecting two mole- 

 cules of urea by an acrylic acid link, with loss of 

 hydrogen and water. Hypoxanthin only differs 

 from the latter in having an atom of oxygen less ; 

 and adenine is the imide of hypoxanthin. If two 

 methyls replace two hydrogen atoms in the imido- 

 group of xanthin it is converted into theobromine ; 

 and if three methyls replace three hydrogen atoms 

 of the same group it will form caffeine. 



The true Proteids or the albuminous substances 

 exist in a living and a non-living form. Our know- 

 ledge of the former, the most important and interest- 

 ing substances, is practically nil. They consist of 

 complex compound molecules, extremely unstable, 

 and in a constant state of change that is, of being 

 constantly pulled down and rebuilt by the processes 

 of metabolism. The complexity of such molecules 

 is seen from the formula C 636 H 1025 N 164 FeS 3 181 which 

 is given as the smallest possible empirical formula 

 for haemoglobin. Under the microscope, living pro- 

 teids have been observed to possess a reducing power 

 not shown by dead proteid. " This and other con- 

 siderations have led to the hypothesis that living 

 proteids partake more of the character of aldehydes, 

 while the dead ones are ke tonic in nature. Some 

 authorities are of opinion that proteids are formed 

 from ammonia and formic aldehyde, as constituents 

 of aspartic aldehyde, 4CHOH + NH 3 = C 4 H 7 N0 2 + 

 2H 2 ; and that by polymerisation, in the presence 

 of sulphuretted hydrogen, we arrive at one of the 

 proposed formulae for albumin, C 72 H 112 N 18 S0 2 2. 



