THE CHEMISTRY OF THE ATMOSPHERE. 97 



mingle intimately in a little while, but will do so at once if 

 shaken or stirred together. So it is with the gases. If in 

 the experiment in 120 we shake the apparatus, the two 

 gases will not require an hour or two to mingle, as is the case 

 when they are still, but will do so at once. So the constant 

 motion of the atmosphere causes the gases that compose it 

 to mingle together most perfectly, so that the carbonic an- 

 hydride, heavy as it is, though constantly produced in vari- 

 ous ways, as you will soon see, at the bottom of the atmos- 

 pheric sea that envelops the earth, is readily diffused 

 throughout that sea though it be fifty miles high. This is 

 not done merely by violent winds, but equally nay, more 

 by the slighter motions which are every where and always 

 going on in the air. 



122. Grotto del Cane. There are some localities where the 

 carbonic anhydride does, however, lie along under the other 

 gases of the atmosphere. Such a locality is the Grotto del 

 Cane in Italy, so called because the layer of gas on the 

 floor of the grotto is only high enough to destroy dogs that 

 enter it cane being the Italian for dog. There are two 

 reasons for this accumulation of carbonic anhydride. One 

 is, that the gas is produced at the locality in very great 

 abundance from some chemical operations in the earth; 

 that is, it is produced so rapidly that it is not all readily 

 diffused. Another is, that the locality is so sheltered as to 

 shut out in some good degree the common agitation of the 

 air. 



123. Carbonic Anhydride in "Wells. Sometimes this gas 

 is generated in wells and deep pits. When this is the case, 

 the diffusion must be slow, the air being confined, and so 

 prevented from being agitated. The gas will therefore ac- 

 cumulate, being mingled with air toward the mouth of the 

 well, but not so at the bottom. If a light then be lower- 

 ed, it will burn more and more dimly as it goes down, and 



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