VI.] 



THE ATMOSPHERE. 



77 



in the tube, as at b, you may catch anything that distils 

 over; and it will be found, at the end of the experiment, 

 that this part of the tube contains metallic mercury. If io8 

 grains of the red powder be heated in a, you may obtain in b 

 I GO grains of the liquid metal; in other words, you have 

 expelled all the matter which has been absorbed from the 

 atmosphere during the process of rusting, and have regained 

 the original weight of quicksilver. The matter which has 

 been thus expelled from the powder by heat need not be 

 lost ; for by attaching to the apparatus a tube c, which dips 



Fig. 20. — Decomposition of red oxide of mercury. 



beneath water in a vessel d, it will be found, on heating the 

 powder in a, that bubbles of gas rise in the water ; and 

 these bubbles may be conveniently collected in the bell-jar 

 E. In this way you obtain a colourless and transparent 

 gaseous body, not to be distinguished by the eye from 

 ordinary air. Yet you have only to plunge a lighted taper 

 into it in order to see at once that you are dealing with 

 something distinct from common air. The taper burns in 

 it with unusual briUiancy ; and even if extinguished before 

 entering the gas, so that only the merest point remains in 

 a state of glow, this glowing point will be rekindled and 



