222 Dr. Tliomson's reply to Berzelius. 



phosphate of soda from the Apothecaries'-hall for a particular 

 purpose, I was astonished to find that when I mixed 7*5 

 grains of the ignited phosphate with a muriatic solution of 6-25 

 grains of calcareous spar and evaporated the mixture to dry- 

 ness, and digested the dry mass in water, this water con- 

 tained a quantity of unprecipitated lime. I found that to 

 precipitate the whole lime, it was necessary to employ 8*5 

 grains of anhydrous phosphate of soda instead of 7'5. From 

 this it is obvious that the acid in the phosphate weighed ^'5 

 and not 3*5. I extracted a quantity of phosphoric acid from 

 eai-th of bones and combined it with soda. 8-5 grains of this 

 salt when anhydrous were still necessary to throw down all 

 the lime from the muriatic solution of6*25 grains of calcareous 

 spar. I made a quantity of phosphoric acid by the slow com- 

 bustion of phosphorus and subsequent digestion in nitric 

 acid. The atomic weight of this acid was also 4*5. 



I think that there exists two different phosphoric acids 

 which have not hitherto been distinguished from each other, 

 one weighing 3*5 and the other 4*5. Stromeyer seems to have 

 encountered the former in his analysis of Cornish hydrous 

 phosphate of iron ( Untermihunger, p. 274) ; and I found it in 

 the phosphate of soda prepared by me many years ago in 

 Edinburgh. I made my phosphoric acid, if I remember right, 

 by dissolving phosphorus in nitric acid; but the atomic weight 

 of the most common phosphoric acid is 4*5. 



I have read over carefully the experiments of Rose on 

 phosphuretted hydrogen gas, and have found nothing in them 

 in the least inconsistent with my experiments on the same 

 gas, which were made so carefully that I cannot doubt their 

 accuracy. Rose's conclusions indeed are inconsistent with 

 mine. But I still think my number for phosphorus, viz. 1*5, 

 right. There are undoubtedly three acids of phosphorus, 

 which must weigh respectively 2-5, 3*5, and 4*5. 



Thei"e is a circumstance connected with the water of cry- 

 stallization in oxalic acid which I find myself unable to account 

 for. I find that 9 grains of the crystals of this acid saturate 

 6 grains of potash and precipitate 6,25 grains of calcareous 

 spar dissolved in muriatic acid without leaving any residue. 

 Hence I conclude, as I have stated in my First F7-hiciples, that 

 these crystals contain half their weight of water. Dr. Prout 

 wrote me, before the publication of my Fhst Principles, that 

 he had uniformly found the crystals of oxalic acid composed of 



Acid 4*5 



Water 3-375 



7-875 



This 



