338 ABOUT COMMON WATER. 



a very heavy gas. It effervesces in soda-water, and it 

 constitutes a portion of the breath exhaled from the 

 lungs. The weight of the gas, as compared with 

 air, may be accurately determined by the chemist's 

 balance. 



But its weight may also be shown in the following 

 way. Let a wide glass shade be turned upside down, 

 and filled with carbonic-acid gas. This is readily done, 

 though when done you do not see the gas. Well, iron 

 sinks in water, because it is heavier than water; it 

 swims on mercury, because it is lighter than mercury. 

 For the same reason, if you blow a soap-bubble and 

 dexterously shake it off, so that it shall fall into the 

 glass shade, it is stopped at the top of the shade, bob- 

 bing up and down, as if upon an invisible elastic cushion. 

 The light air floats on the heavy gas. Almost any 

 other acid, poured upon chalk or marble, liberates the 

 carbonic acid. Its grasp of the lime is feeble, and 

 easily overcome. When we dissolve and mix a com- 

 mon soda-powder, the tartaric acid turns the weaker 

 carbonic acid out of doors. 



Many natural springs of carbonic acid have been 

 discovered, one of which I should like to introduce to 

 your notice. In the neighbourhood of the city of 

 Naples there is a cave called the Grotto del Cane, a 

 name given to it for a curious and culpable reason. 

 During one of the eruptions of Vesuvius I paid a visit, 

 in company with two friends, to Naples, and went to 

 see, among the other sights of that wonderful region, 

 the Grotto of the Dog. At a place adjacent we met a 

 guide and some other visitors. At the heels of the 

 guide was a timid little quadruped, which, for the time 

 being, was the victim that gave the cave its name. We 

 could walk into the cave without inconvenience, know- 

 ing, at the same time, from the descriptions we had 



