366 Dr. Thomson on the [Nov* 



red heat in a balanced platinum crucible. The sulphate of 

 barytas obtained by this process, gave the quantity of sulphuric 

 acid in the sulphate of zinc with sufficient exactness. But 

 there were two sources of inaccuracy which prevented me frdm 

 obtaining exactly the oxide of, zinc. A portion of the 'zinc 

 was usually sublimed during the exposure of the acetate to tt 

 red heat. When the oxide of zinc obtained was dissolved in 

 an acid, a little charcoal from the decomposed acetici^oid 

 usually remained behind. 'i\ni\n ^^ 



(2.) The sulphate of zinc was dissolved in water, and the 

 solution mixed with a sufficient quantity of oxalate of ammonia 

 to throw down all the zinc. The oxalate of zinc was collected 

 on a double filter; and the liquid which passed through, 

 together with that employed in washing the oxalate of zinc, was 

 evaporated to dryness, and the residue redissolved in water, to 

 obtain any oxalate of zinc which might not have been deposited^ 

 at first. The oxalate of zinc thus obtained was dried on the!' 

 filter and weighed. A portion of it was exposed to a red heat^' 

 and the loss of weight determined. From this, the whole oxidei( 

 of zinc contained in the salt was deduced. 



This mode of analysis came much nearer the truth than the 

 preceding. Indeed, the oxide of zinc calculated from the: 

 oxalate would be exact; but this method could not be emwtr 

 ployed, because it required the previous knowledge of the vei^'v 

 thing which was under investigation. The oxide of zinc ob-*^' 

 tained by exposing the oxalate to a red heat, always was below 

 the truth. The deficiency, when 181*25 grains of sulphate 

 of zinc had been employed, varied in diffigrent trials from 1 

 grain to 2^ grains, and it was never less than one grain. I 

 ascribe this to a httle zinc sublimed during the apphcation of' 

 the heat. It was not owing to any of the zinc being in the 

 metallic state ; for I digested the residue in nitric acid without 

 any alteration in the total weight.' nf^'^ tn':)iorftif>' bsbfvoiq ^^j^F» 



4. Foiled in both of these modes of experimerntihg,!* had re- 

 course to the decomposition of sulphate of zinc by carbonate 

 of soda. This method came sufficiently near the truth to sa- 

 tisfy myself completely respecting the true weight of oxide of 

 zinc contained in a given weight of sulphate. Perhaps the 

 most instructive analysis I can select will be the preliminary 

 one, by which I determined the circumstance necessary to be 

 attended to in order to obtain the whole ofu the oxide nof zinc 

 from a given weight of sulphate. »i iii> iiaii v6 ( 



( I .) 1 commonly took 90*625 grains (five atoms) of crystal- 

 lized sulphate of zinc ; the smallest weight which 1 employ is 

 0*01 ^rain. I weighed out commonly 90*63 grains of sulphate 

 of zin6, and afterwards checked this analysis by another, in 

 which the weight of the salt was 90*62 grains. These two 



