CMBMICAL HISTORY OF A CANDLH 



151 



<an healthily and safely breathe ; for it is just as important 

 to make the oxygen right for us to breathe, as it is to make 

 the atmosphere right for the burning of the fire and the 

 candle. 



But now for this atmosphere. First of all, let me tell 

 you the weight of these gases. A pint of nitrogen weighs 

 10 4-10 grains, or a cubic foot weighs I 1-6 ounces. That is 

 the weight of the nitrogen. The oxygen is 

 heavier: a pint of it weighs n 9-10 grs., and a 

 cubic foot weighs I 3-4 oz. A pint of air weighs 

 about 10 7-10 grs., and a cubic foot I 1-5 oz. 



You have asked me several times, and I am 

 very glad you have, "How do you weigh gases ?" 

 I will show yout it is very simple, and easily 

 done. Here is a balance, and here a copper bottle 

 made as light as we can consistent with due 

 strength, turned very nicely in the lathe, and 

 made perfectly air-tight, with a stop-cock, which 

 we can open and shut, which at present is open, 

 and, therefore, allows the bottle to be full of 

 air. I have here a nicely-adjusted balance in 

 which I think the bottle, in its present condition, 

 will be balanced by the weight on the other side. 

 And here is a pump by which we can force the 

 air into this bottle, and with it we will force in 

 a certain number of volumes of air as measured 

 by the pump. [Twenty measures were pumped 

 in.] We will shut that in and put it in the balance. 

 See how it sinks; it is much heavier than it was. By what? 

 By the air that we have forced into it by the pump. There 

 is not a greater bulk of air, but there is the same bulk of 

 heavier air, because we have forced in air upon it. And 

 that you may have a fair notion in your mind as to how 

 much this air measures, here is a jar full of water. We 

 will open that copper vessel into this jar, and let the air 

 return to its former state. All I have to do now is to 

 screw them tightly together, and to turn the taps, when 

 there, you see, is the bulk of the twenty pumps of air which 

 I forced into the bottle ; and to make sure that we have been 

 quite correct in what we have been doing, we will take the 



FIG. 79 



