Every gas in this table is perfectly transparent to light, that is to say, all waves within the limits of the visible spectrum pass through it without obstruction; but for the waves of slower period, emanating from our heated plate of copper, enormous differences of absorptive power are manifested. These differences illustrate in the most unexpected manner the influence of chemical combination. Thus the elementary gases, oxygen, hydrogen and nitrogen, and the mixture atmospheric air, prove to be practical vacua to the rays of heat; for every ray, or more strictly speaking, for every unit of wave motion, which any one of them is competent to intercept, perfectly transparent ammonia intercepts 5460 units, olefiant gas 6030 units, while sulphurous acid gas absorbs 6480 units. What becomes of the wave motion thus intercepted? It is applied to the heating of the absorbing gas. Through air, oxygen, hydrogen, and nitrogen, on the contrary, the waves of ether pass without absorption, and these gases are not sensibly changed in temperature by the most powerful calorific rays. The position of nitrous oxide in the foregoing table is worthy of particular notice. In this gas we have the same atoms in a state of chemical union, that exist uncombined in the atmosphere; but the absorption of the compound is 1800 times that of the air.
