THE GREAT LONDON CHEMISTS 61 



chemists. Graham proved that orthophosphoric acid 

 consists of a compound of the anhydride, P 2 5 , with 

 three molecules of water, and that each molecule is 

 capable of replacement by the oxide of such a metal as 

 sodium ; that pyrophosphoric acid may be regarded as 

 composed of a molecule of anhydrous phosphoric acid 

 with two molecules of water, each of which is replaceable 

 by an oxide; and that metaphosphoric acid is to be 

 represented as a compound of one molecule of anhydride 

 with one molecule of .water. The general term, which 

 came to be used for this behaviour of acids was basicity, 

 and an acid was termed monobasic, dibasic, or tribasic, 

 according as it was capable of uniting with one, two, or 

 three molecules of base; yet it might contain the same 

 anhydrous oxide in each case. These views of Graham's 

 made it possible to account for the fact, at that time most 

 mysterious, that on mixing nitrate of silver, with its 

 neutral reaction, with alkaline phosphate of sodium, an 

 acid liquid was the result. These experiments of Graham's 

 paved the way for the later theory, that acids are salts of 

 hydrogen. In Graham's language, the three phosphoric 

 acids were ' terphosphate, biphosphate, and phosphate of 

 water ' ; for he understood by the term ' phosphoric acid ' 

 what we nowadays name phosphoric anhydride. The word 

 phosphate, however, is now applied to the group P0 4 , 

 and hence the name phosphates of hydrogen. Graham 

 was the first to recognise that (to quote his own words) 

 ' when one of these compounds (the phosphoric acids) 

 is treated with a strong base, the whole or a part of the 

 water is supplanted, but the amount of base in combina- 

 tion with the acid remains unaltered! We should now 

 say, ' the whole or a part of the hydrogen of the acid is 

 supplanted, but the total number of atoms of hydrogen 

 plus metal in the salt remains'unaltered.' 



Continuing the train of ideas aroused by his researches 



