206 SCIENCE PROGRESS 



was one of them and the others are related to it ; another 

 term was that of alloxuric bases, but finally the term purine 

 bases was definitely adopted, for they are derivatives of a 

 ringed nucleus called purine by Emil Fischer. 



These bases are four in number; their names and formulae 

 are as follow : 



Purine C 5 H 4 N, 

 f Hypoxanthine (monoxypurine) C 5 H 4 N 4 

 Purine bases ) Xanthine (dioxypurine) C 3 H,N 4 2 



J Adenine (amino-purine) CsH^N^NR.) 

 ^-Guanine (amino-oxypurine) C 5 H 3 N 4 0(NH.,) 



and at first it was supposed there were four different nucleic 

 acids, each of which yielded a different base. 



Later other bases were separated out and named cytosine, 

 thymine and uracil. Their composition and constitution finally 

 were unravelled, and they were shown to be derivatives of 

 another ringed nucleus named pyrimidine ; hence they are 

 called the pyrimidine bases. 



The result of this work was to show that the cleavage 

 products of the nucleic acids may be tabulated into : 

 i. Phosphoric acid. 



2. A carbohydrate. 



3. Purine bases. 



4. Pyrimidine bases. 



A large amount of information was in this way placed at the 

 disposal of physiologico-chemical workers, but a glance at 

 the text-books of that date (six to ten years ago) will show 

 that no clear conception of the construction of nucleic acid 

 was then possible. Each worker or writer interpreted the 

 mass of facts in a different way, and the net result was chaos. 

 The difference observed in the composition and cleavage 

 products of nucleic acids obtained from various sources was 

 still interpreted as due to differences in the proportion in 

 which the cleavage products were combined. It need hardly 

 be said that if such a view were correct, and each nucleus 

 had its own specific nucleic acid, the complexity of the subject 

 would have been well-nigh insuperable. Happily, however, 

 this is not really the case. As the years go by, and better 

 methods for the separation and purification of nucleic acid 

 •are introduced, the difficulties are being cleared up, and 

 nucleic acids previously supposed to be different are now to 



