14,8 Analif SIS of Scientific Books. 



In the edition of the System published in the autumn of 1817, 

 Dr. Thomson says, " It will appear, in a subsequent part of this 

 section, that phosphorous acid is composed of 1.5 phosphorus, 

 and 2 oxygen. Hence its constituents are, 



Phosphorus 100 



Oxygen 133}" 



The subsequent part of the section to which he alludes is the 

 following ; " So that the phosphorus in phosphurctted hydrogen, 

 combines with either ^ volume, or with 1 volume of oxygen gas. 

 In the first place, 1 suppose that the hypophosphorous acid is 

 formed. In the second case, phosphorous acid*." Now as he makes 

 the volume of phosphorous vapour 0.8328, we have the proportion 

 of 0.8328 to 1.1111, or 100 to 133.4 as above. Observe, this 

 is the Doctor's last decision, prior to Sir H. Davy's paper in 

 the Transactions for 1818. Here, however, we find the follow- 

 ing statement : " If it be supposed a simple compound of oxygen 

 and phosphorus, the series of proportions in the acids of phos- 

 phorus will be 



I'losp. Oxyg. Phosp. Oxyg. 



Hypophosphorous acid, 45 15 3:1 



Phosphorous acid, 45 30 1.5 : 1 



Phosphoric acid, 45 60 1.5 : 2" 



Compare with the above, the following table in Dr. Thomson's 



5th edition : •' Thus the three acids of phosphorus are composed 



as follows : 



Phosp. Oxyg. 



Hypophosphorous acid, 1.5 1 



Phosphorous acid, 1.5 2 



Phosphoric acid, 1.5 3t" 



In his 6th edition, we fii^d the following bold statement: 

 " The analysis of phosphorous acid given by Davy, in his 

 paper published in the Philosophical Transactions for 1818, 

 exactly agrees with mine, and serves, of course, to confirm it J." 

 " And the weight of an atom of phosphorous acid is 2.5." We 

 suppose there must be an error of the press, and for " confirm," 

 we should read " confute." 



One would imagine the atom of phosphoric acid fixed in the 

 Doctor's mind ; yet he devotes a whole page of his present edition 

 to shew, that, the analyses by Berzelius of the phosphates of lead, 

 barytes, soda, and lime, make the atomic weight of the acid 

 4.5 ! And from the same authority, he deduces the atomic weight 

 of phosphorous acid to be 3.5. In such contradiction with it- 

 self, is his article on phosphorus. " It must be allowed," 

 says he, " that the evidence advanced by Berzelius has a 

 very imposing aspect. But I think my mode of determin- 

 ing the composition of phosphorous acid is so much more 



• System, 5th edition, I, -27i. f Ibid., p- 266. 



* Ibid., 6ch edition, \, p. 363. 



