260 Dr. Thomson's Experiments to determine the Weight [April, 



quantity is only one-fourth of an atom of water, and mine only 

 one-eighth of an atom of water, less than 5*625 grs. But as 

 neither three-fourths nor seven-eighths of an atom of water can 

 enter into combination, we are entitled to consider the true 

 quantity of water present as 5 atoms, or 5*625 grs.; conse- 

 quently, the protoxide of manganese present must weigh exactly 

 4*5 grs. Tims we get 4-5 for the weight of protoxide, and 3*5 

 for the weight of manganese in the metallic state. 



Thus we see that the atomic weights of iron and manganese 

 are the same. 



V. Nickel 



I am not certain that any chemist has taken the trouble to 

 make an accurate analysis of sulphate of nickel. It is a salt 

 which crystallizes in beautiful four-sided prisms, and is not 

 altered by exposure to the atmosphere. I find that 17*125 grs. 

 of this salt, when dissolved in water, and mixed with a solution 

 of 13*25 grs. of chloride of barium, let fall sulphate of baiytes; 

 and the clear liquid, after the precipitate has fallen to the bot- 

 tom, is neither affected by muriate of barytes, nor by sulphate 

 of soda ; consequently, it neither contains sulphuric acid nor 

 barytes. It is clear from this, that 17*125 grs. of the salt con/- 

 tain exactly 5 grs. of sulphuric acid. When 17*125 grs. of 

 crystallized sulphate of nickel are exposed to a red heat, they 

 sustain a loss of weight varying from 7*8 grs. to 8 grs. Now 

 the weight of 7 atoms of water is 7*875, which agrees almost 

 exactly with the loss of weight found by experiment. It is 

 obvious from this that 17*125 grs. of this salt are composed as 

 follows : 



1 atom sulphuric acid = 5*0 



1 atom protoxide of nickel = 4*25 



7 atoms water = 7*875 



17*125 



Thus it appears that protoxide of nickel weighs 4*25 ; conse- 

 quently the weight of an atom of nickel must be 3*25. 



VI. Cohalt. 



The difficulty of obtaining this metal in a state of pm-ity has 

 hitherto retarded the exact determination of its atomic weight. 

 I took a quantity of oxalate of cobalt, which I had purified by 

 Laugier's process, exposed it to a red heat in a platinum cruci- 

 ble, Jind then dissolved it in sulphuric acid. By the requisite 

 concentration, the sulphate of cobalt is obtained in small red 

 prismatic crystals, which are not sensibly altered by exposure 

 to the air. They may be exposed to an incipient red heat with- 

 out losing any of their acid, provided we take care to raise the 

 heat slowly, and not to carry it too far. By this method I was 

 able, after a little practice, to deprive them entirely of their 



