of Nickel and its Oxides. 325 



tains exactly five grains of sulphuric acid. Dr. Turner has 

 objected to this conclusion, that I employed 13'25 grains 

 of chloride of barium instead of 13 grains, the quantity 

 containing the exact proportion of barium that is necessary 

 to saturate 5 of sulphuric acid. It would seem at first 

 sight, that my experiment indicated the presence of 5*096 

 of sulphuric acid instead of 5. But I find that in practice 

 we must always employ 13*25 of chloride of barium to 

 throw down 5 of sulphuric acid. If we employ less, the 

 whole sulphuric acid is not thrown down. In consequence 

 of this curious fact, I found it impossible to determine the 

 atomic weight of barytes by mixing sulphate of potash and 

 chloride of barium. The result always gave 9*75 for the 

 atomic weight of barytes. I succeeded in obtaining 9*5 

 only when I substituted carbonate of barytes for sulphate, 

 and employed re-agents, which were dissipated by a red 

 heat. Many an experiment on this subject has been made 

 by my practical pupils, and some of the most accurate 

 among them have been so much struck with it as to be 

 inclined to adopt 9*75 for the atomic weight of barytes. 

 But I satisfied myself many years ago (as I have elsewhere 

 stated) that 9*5 is the real number; but, that owing to 

 some curious combinations which take place, we never can 

 precipitate the whole sulphuric acid from a solution without 

 employing a slight excess of barytes. 



The above experiments (which any person may easily 

 repeat) leave no doubt on my mind, that 17*125 grains of 

 pure crystallized sulphate of nickel contain exactly 5 grains 

 of sulphuric acid. 



The determination of the water of crystallization of this 

 salt is a very difficult thing. For if we heat the salt too 

 high, we drive off'not merely the water, but also a portion 

 of the acid. The heat should be applied very slowly and 

 equably, and it should be raised higher than 600°, but not 

 so high as a red heat. If the salt after this exposure be 

 still soluble in water, it is a proof that none of the acid has 

 been driven off. In a very cautious experiment I drove 

 off 7*8 grains from 17*125 of the crystals, and the salt was 

 still soluble in water. In another, I drove off 8 grains ; but 

 the salt did not dissolve completely in water. From these 

 trials 1 concluded, that the water of crystallization lies 



