&ADIATION. 29 
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 un- 
expected manner the influence of chemical combination, 
Thus the elementary gases, oxygen, hydrogen, and nitro- 
gen, and the mixture atmospheric air, prove to be prac- 
tical vacua to the rays of heat; for every ray, or, more 
strictly speaking, for every unit of wave-motion, which 
any one of them intercepts, perfectly transparent ammonia 
intercepts 5,460 units, defiant gas 6,030 units, while sul- 
phurous acid gas absorbs 6,480 units. What becomes of 
the wave-motion thus intercepted? It is applied to the 
heating of the absorbing gas. Through air, oxygen, hydro- 
gen, and nitrogen, 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 tLe 
atmosphere; but the absorption of the compound is 1,800 
times that of air. 
5. Formation of Invisible Foci. 
This extraordinary deportment of the elementary gases 
naturally directed attention to elementary bodies in other 
states of aggregation. Some of Melloni's results now at- 
tained a new significance. This celebrated experimenter 
had found crystals of sulphur to be highly pervious to 
xadiant heat; he had also proved that lampblack, and 
black glass (which owes its blackness to the element car- 
ibon) were to a considerable extent transparent to calorific 
rays of low refrangibility. These facts, harmonizing so 
istrikingly with the deportment of the simple gases, sug- 
gested further inquiry. Sulphur dissolved in bisulphide of 
<earbon was found almost perfectly diathermic. The dense 
.and deeply-colored element bromine was examined, and 
found competent to cut off the light of our most brilliant 
flames, while it transmitted the invisible calorific rays with 
icxtreme freedom. Iodine, the companion element of bro- 
mine, was next thought of, but it was found impracticable 
