t370 Dr, Thomson on the [Nov. 



lowed to determine the quantity of sulphuric acid. The process 

 has been already given in the Annals of Philosophy ^ vol. i. 

 p. 246, New Series. It consisted ii^ dissolving l^-l25 grains 

 of sulphate of zinc, and 13*25 of chloride of barium, in as little 

 water as possible, mixing the solutions ;' and afler the sulphate 

 of barytes has subsided, testing the clear supernatant liquid 

 with muriate of barytes and sulphate of soda, and finding that 

 the liquid is affected by neither. From this experiment I con- 

 cluded, that 18-125 grains of sulphate of zinc contain exactly 

 five grains of sulphuric acid. ' , 



I made an experiment on purpose to ascertaii;i hp,w ismajl a 

 portion of sulphuric acid could be detected in water by this 

 auethod. One grain of glauber salt being dissolved in 12000 

 grains of water, I found that the solution was distinctly pre- 

 cipitated by muriate of barytes. Now, one grain of glauber 

 salt contains rather less than -fth grain of sulphuric acid. The 

 liquid in which 1 had dissolved the 18-123 grains of sulphate 

 of zinc and 13.25 grains of chloride of barium did not amount 

 to so much as 500 grains ; hence had so much as -g^th of a 

 grain of sulphuric acid remained in solution, it would have been 

 rendered visible by the muriate of barytes dropt into the liquid. 

 I was sure, therefore, from this experiment, that the sulphuric 

 acid in 18*125 grains of sulphate of zinc is not less than five 

 grains, and not so much as 5*01 grains. There could be no 

 hesitation in concluding that the exact quantity was five, espe- 

 cially as this is the atomic weight of sulphuric acid. 



Knowing that 18 125 grains of sulphate of zinc contain five 

 grains of sulphuric acid, and that the oxide of zinc is not less 

 than 5-245 grains, and the water not so much as 8*16 grains ; and 

 knowing farther, that the water in crystallized salts constitutes 

 a certain number of atoms, and that the atom of water weighis 

 1*125; there was no longer any difficulty in determining the true 

 atomic weight of oxide of zinc, and the exact quantity of \% 

 contained in 18*125 grains of sulphate of zinc. 



Let the atom of oxide of zinc = x, and let 18*125 sulphate of 

 zinc be a compound of 



1 atom sulphuric acid = 5 

 1 atom oxide of zinc = x 

 7 atoms water = 1*125 y 



We have 5 + x + 1*125^/ = 18*125 



We know that x is not less than 5*245, and that y is more than 

 6, and not more than 7. If we make y = 7, then we have 



5 + 7*875 ■\- X = 18*125, and consequently 

 a; = 18*125 - 12*875 = 5*25. 



Nor can any numbers be substituted for ?/ and x consistently with 

 ^e preceding experiments, except 7 and 5*25. 



