CHEMICAL RAYS. 251 



ploy oxygen, hydrogen, or nitrogen. With hydrogen curi- 

 ous effects are observed, due to the sinking of the clouds 

 through the extremely light gas in which they float. They 

 illustrate, but do not prove, the untenable notion of those 

 who say that the clouds of our own atmosphere could not 

 float if the cloud-particles were not little bladders instead 

 of full spheres. Before you is a tube filled with the nitrite- 

 of-amyl vapor, which has been carried into the tube by 

 hydrogen gas. On sending the beam through the tube a 

 delicate bluish-white cloud is precipitated. A few strokes 

 of the pump clear the tube of this cloud, but leave a resi- 

 due of vapor behind. Again, turning on the beam we have 

 a second cloud, more delicate than the first. This may be 

 done half a dozen times in succession. A residue of vapor 

 will still linger in the tube sufficient to yield a cloud of ex- 

 quisite delicacy, both as regards color and texture. 



Besides the nitrite of amyl, a great number of other 

 substances might be employed, which, like the nitrite, have 

 been hitherto not known to be chemically susceptible to 

 light. This is, in fact, a representative case. One point 

 in addition I wish to illustrate, chiefly because the effect is 

 the same in kind as one of great importance in nature. Our 

 atmosphere contains carbonic-acid gas, which furnishes food 

 to the vegetable world. But this food, as many of you 

 know, could not be consumed by plants and vegetables 

 without the intervention of the sun's rays. As far as we 

 know, however, these rays are powerless upon the free car- 

 bonic acid of our atmosphere ; the sun can only decompose 

 the gas when it is absorbed by the leaves of plants. In the 

 leaves the carbonic acid is in close proximity with sub- 

 stances ready to take advantage of the loosening of the 

 molecules by the waves of light. Incipient disunion being 

 introduced by the solar rays, the carbon of the gas is seized 

 upon by the leaf and appropriated, while the oxygen is dis- 

 charged into the atmosphere. 



