12 Dr. Thomson on the true Atomic Weight of [Jan. 



portion of the clear liquid was drawn off and examined. It 

 yielded no precipitate when mixed with nitrate of lead or muriate 

 of lime ; and, therefore, contained no phosphoric acid in solu- 

 tion ; neither was it affected when sulphate of soda was dropped 

 into it— a proof that it was equally free from lead. We see then 

 that the phosphoric acid in 7*5 grs. of anhydrous phosphate of 

 soda is exactly neutralized by the oxide of lead in 20*75 grs. of 

 nitrate of lead. Therefore an atom of phosphoric acid weigh* 

 3*5, and phosphate of lead is composed of 



Phosphoric acid . . 3*5 20 .... 100 



Oxide of lead 14-0 .... 80 .... 400 



17^ 100 



Here then is the source of Berzelius's mistakes. He makes 

 the composition of phosphate of lead to be 



Phosphoric acid 100 



Protoxide of lead 314 



I do not know the reason of this difference. It must, I think, 

 be owing either to a mixture of two phosphates of lead, or to the 

 formation of a compound different from neutral phosphate of 

 lead. I obtained a result approaching to that of Berzelius, when 

 I attempted to determine the composition of phosphate of lead 

 by direct experiments. But be the cause of the error what it 

 may, there can be no doubt entertained of its existence, at least 

 by any person who will take the trouble to repeat the experiment 

 which 1 have just described. 



We see that the weights of the atom of phosphoric acid and 

 of hme are exactly equal. Hence neutral phosphate of lime ig 

 composed of 



Phosphoric acid 50 



Lime 50 



100 



When I first attempted to determine the weight of an atom of 

 phosphoric acid, I had recourse to salts of lime, knowing the 

 perfect insolubility of phosphate of hme in water. I dissolved 

 determinate weights of^ carbonate of lime in muriatic acid^ evapo- 

 rated the solution to dryness, and mixed it with a determinate 

 weight of phosphate of soda ; but I soon found that it is impos- 

 sible to precipitate Hme completely from a muriatic acid solution 

 of lime (though perfectly neutral) by means of phosphate of soda. 

 The solution of muriate of soda has the property of holding phos- 

 phate of Hme in solution, and the presence of lime is always 

 indicated in the clear solution by means of oxalate of ammonia, 

 which throws down a copious precipitate of oxalate of Hme. I 

 have no doubt that this solubility of phosphate of lime in solu- 



