Thomson's Sj/stem of Cheiidstri/. 147 



afe reprinted without alteration, and contain nothing remarkable 

 any way. To phosphorus in the fifth section he devotes nearly 

 twenty-four pages, of which about five are atomic altera- 

 tions. The quantity of sulphuric acid, 83^, which he prescribes 

 for decomposing 100 parts of calcined bones, is much too great, 

 and will spoil the subsequent process for procuring phosphorus. 

 And with regard to forming, as he directs, a phosphate of lead, 

 with nitrate of lead, as a step in the operation, no practical 

 chemist could afford to practise it. In his view of the acids 

 of phosphorus, which is reprinted in the second volume, 

 secundum artem, he has contrived to make himself appear the 

 hero of the scene, though it is one in which he has played 

 a very subaltern part. He has suppressed entirely the 

 long succession of blunders into which he plunged, and from 

 which he was extricated by Sir H. Davy's rebearches published 

 in the Philosophical Trayisactions for 1818. Well may the Doctor 

 say, " The weight of an atom of phosphoric acid, has cost me 

 first and last, a good deal of trouble*." 



In 1816, he presented a paper on the subject to the Royal 

 Society, which after being read, was withdrawn. Full of 

 its importance, however, at the time, he hastened to publish 

 in his Annals for April, a detail of its contents, among which 

 we find the atom of phosphoric acid, declared to be 3.634. 

 Meanwhile he sets to work on phosphuretted hydrogen, and 

 deduces from it, the atomic weight of the acid to be 3.5, 

 which new decision, he publishes in his Annals for August 

 1816. But lo ! in January following, we find hira affirming 

 that 4.5 is the true atomic weight of phosphoric acid, and 

 that 3.5 is undoubtedly wrong, and must be abandoned. 

 In the subsequent October, the 5th edition of his System comes 

 forth, with a long array of proofs, shewing that 4.5 is the true 

 atom of phosphoric acid. Sir H. Davy's decisive experiments, 

 on the subject, being read, however, before the Royal Society in 

 1818, made him reflect a little on his contradictions ; subsequent 

 to which time, he seizes on the number 3.5 fixed by Sir H. Davy, 

 and finding it, among his former guesses, claims it as his own 

 prior discovery. 



The following sentence shews the Doctor's incompetency as 

 an experimentalist. " Unless the flask," says he, " be very 

 completely exhausted, indeed, of common air, combustion 

 takes place when the phosphuretted hydrogen gas is let into it." 

 This confession betrays poverty of invention, and ignorance of 

 the methods previously practised in such cases. If the Doctor 

 will consult Sir H. Davy's Bakerian Lecturer on the Alkaliyie 

 Metals, he will find this philosopher filling his flask with hy- 

 drogen, and ihen exhausting that gas, in order to get entirely rid 

 of the atiuosphfci iyal oxygen. 



y 



*■ Alinnh nf Philosnphy^ for .I»nu»n' 18-1, p. f. 



L 2 



