260 Dr, Thomsoii*s Answer to the Review of the [ApRji, 



ncid was by Lavoisier (Mem. Paris, 1777, p. 65 ; 1780, p. 343 ; 

 and 1783, p. 416). His experiments were continued 12 years, 

 and his ultimate result was, that the acid is composed of two 

 phosphorus and three oxygen. 



Davy, in his Elements of Chemical Philosophy (p. 286), says, 

 that when a grain of phosphorus is strongly heated in oxygen 

 gas, it absorbs four cubic inches and a half of the gas. Ihifi 

 statement coincides with the original one of Lavoisier. Whe- 

 ther it was the result of experiment on the part of Davy does 

 not appear. 



M. Rose endeavoured to determine the question in 1806 

 (Gehlen's Jour. ii. 309, Second Series), by acidifying phosphorus 

 by means of nitric acid, and then saturating the acid with oxide 

 of lead. He deduced from his experiments that the acid is a 

 compound of 100 phosphorus + 114*75 oxygen: 50 grains of 

 phosphorus yielded, when thus treated, 481 grains of phosphate 

 of lead. But as phosphate of lead is composed of 14 oxide of 

 lead -r 3*5 phosphoric acid, it is obvious that 481 grains of it 

 contain only 96*2 grains of phosphoric acid. Hence the true 

 deduction from Rose's experiments is, that phosphoric acid is a 

 compound of 100 phosphorus + 92*4 oxygen. 



In the year 1816, a paper of mine on the composition of 

 phosphoric acid, and on various phosphates, was read before 

 the Royal Society. I had repeated Rose's experiments with 

 great care, but found the method not to be relied on. I, there- 

 fore, drew my conclusions from the gas absorbed, when a given 

 weight of phosphorus is burnt in retorts filled with common air. 

 The result of my trials was that one grain of phosphorus, when 

 thus burnt, absorbs 3*66 cubic inches of oxygen gas. Hence I 

 concluded that the acid is composed of 100 phosphorus -f- 123*5 

 oxygen. This result was not quite correct. The error amounted 

 to nearly l-13th of the whole, or about l-4th of a cubic inch, by 

 which the oxygen gas absorbed was too small. The error was 

 owing to the extreme difficulty of burning all the phosphorus, or 

 of weighing with accuracy the unburnt portion. 



Soon after this, a set of experiments on the same subject was 

 published by M. Dulong. His result almost agreed with mine. 

 According to him, phosphoric acid is composed of 100 phospho- 

 rus -f- 124*8 oxygen. 



A few months later, Berzelius favoured the chemical world 

 with the result of his own experiments on the subject. He 

 found the acid composed of 100 phosphorus -f 128*17 oxygen. 

 I do n6t s6e any reason why I should be ashamed of my expe- 

 riments. Compared with the preceding statements of Lavoisier, 

 Davy, and Rose, they are exceedingly accurate. And though 

 Dulong and Berzelius, who followed me, have approached 

 somewhat nearer the truth, yet they were unable to reach it. 

 The priJQcipal object of my paper was to show that one of 



