PHOSPHORIC ACIDS 143 



with gelatin to form an insoluble compound (34), 100 parts of 

 gelatin uniting, according to this observer, with 3.6 parts of 

 metaphosphoric acid. Graham suggested that the difference be- 

 tween the behavior of metaphosphoric and other phosphoric 

 acids in this respect might be connected with the possibly col- 

 loidal character of the latter.* This suggestion has more recently- 

 been reiterated by Mylius (82). There appears, however, no 

 adequate reason for adopting such a conclusion. Metaphosphoric 

 acid differs from orthophosphoric acid in other respects, and these 

 differences are not attributable to the " colloidal" character of 

 its solutions. As Graham pointed out, the low equivalent of 

 metaphosphoric acid which combines with gelatin suggests that 

 the metaphosphoric acid enters into the compound in a crystal- 

 loid and not a colloidal (polymerized) form. Colloidal silicic acid 

 combines with gelatin in its colloidal form and the compound con- 

 tains nearly equal weights of gelatin and of silicic acid (Graham, 

 loc. cit., p. 206). 



The compounds of metaphosphoric acid with the proteins 

 were at one time regarded with peculiar interest on account 

 of their supposed identity with the nucleo-proteins (69) (74). 

 Pohl (96) and Kossel (61), however, showed that these compounds 

 differ fundamentally from the true nucleins in that, on hydrolysis, 

 they yield no purin bases. Thanks to the work of Miescher, 

 Kossel and their pupils, it has now been shown that the nucleins 

 are not protein salts of phosphoric or of metaphosphoric acid 

 but of a complex substituted phosphoric acid, nucleic acid (6) . 



The compounds of the protein with metaphosphoric acid 

 appear, as a rule, to be unstable save in the presence of excess 

 of the acid (Graham, loc. cit., Malfatti, loc. cit.) ; if this excess be 

 removed by washing then the compound undergoes decompo- 

 sition, liberating metaphosphoric acid. Progressive removal of 

 the free acid by washing, therefore, results finally in complete 

 decomposition of the product. Conversely, in the presence of 

 a variable excess of metaphosphoric acid the quantity which is 

 bound by a protein is greater the greater the excess (28). 



Bechhold (14) has described compounds of egg-albumin with 



* "It will be an interesting inquiry whether metaphosphoric acid is a 

 colloid, and enters into the compound described in that character, or is a 

 crystalloid, as the small proportion and low equivalent of the acid would 

 suggest," loc. cit. page 221. 



