THE BOOK OF THE LANTERN. 193 



the other oxygen. It is possible to elaborate this slide by 

 crowning the two terminals with tiny inverted test tubes, 

 filled with the acidulated liquid. In this case the bubbles 

 of gas displace the contained water in the tubes, the hydro- 

 gen tube being readily distinguished by being emptied of 

 water at double the rate of the tube devoted to the oxygen 

 gas. This proves in a very direct manner the composition 

 of water, which consists of two volumes of hydrogen to 

 one of oxygen. 



In order to show the generation of hydrogen gas alone, a 

 still more simple arrangement can be adopted. The electric 

 battery is not used for this experiment. A few pieces of 

 granulated zinc are dropped into the tank of acid water, 

 when bubbles of hydrogen will be rapidly given off, their 

 downward descent upon the screen giving a very peculiar 

 effect. 



In like manner carbonic acid gas can be generated by 

 using a few pieces of marble instead of the zinc, aud sub- 

 stituting for the sulphuric acid water which has been acidu- 

 lated with hydrochloric acid. We can also easily show 

 that one of the products of the lungs is this same carbonic 

 acid gas. In this case the tank must be filled with lime 

 water, which will remain perfectly clear until it is blown into 

 fro n the lungs by means of a tiny glass tube, when bubbles 

 of air will rise from the water, and the liquid will rapidly 

 become cloudy, proving that the carbonic acid from the 

 lungs has formed carbonate of lime, or common chalk, in 

 the water. 



It will be noticed that in all tank experiments it is 

 necessary that the lantern stage should be open at the top 

 



