OXYGEN AND HYDROGEN GASES. 



which are requisite, in order to enable the rea- 

 der to judge of the justice of the conclusions 

 which are to be drawn. Some of the details 

 may appear needlessly minute ; but I must re- 

 quest the reader's attention to them, because 

 they constitute* the foundation of the whole fa- 

 bric which I mean to build. 



The zinc of commerce does not answer for 

 the experiments which I am going to describe. 

 It is never quite free from carbon, which has a 

 material effect in increasing the specific gravity 

 of the hydrogen gas, obtained by dissolving it 

 in sulphuric acid. It contains, likewise, small 

 quantities of several other metals, and does not 

 dissolve completely in sulphuric acid. The 

 zinc which I used, was obtained by exposing 

 common zinc to a white heat in a stone-ware 

 retort, luted to a receiver nearly filled with wa- 

 ter. At this temperature, the zinc is sublimed, 

 and freed from all its impurities, except a trace 

 of cadmium too minute to occasion any sensible 

 error in the result of the experiments. The 

 zinc thus distilled over, should be melted in a 

 crucible, and poured upon the surface of a clean, 

 smooth sandstone. We thus obtain it in a thin 

 sheet, which can be easily broken into small 

 pieces to suit the object in view. 



21-25 grains of this zinc were introduced into 



tion of oxide 



of zinc. a small green glass retort, the weight of which 

 had been previously determined. As much 



