RADIATION. 297 



amounts of wave motion absorbed liv the respective gases, the quant'.ty inter- 

 cepted by atmospheric air being- taken as unity. 



Badiation throiujh gases. 



Name of gas. Relative absorption. 



Nitric oxide 1 .590 



Nitrous oxide 1,SG0 



Sulphide of liydrogeu 2,100 



Anuuunia 5,4G0 



Oletiant gas 6,030 



Sulphurous acid 6,480 



Name of gas. Relative absorption. 



Air 1 



Ox^-gen 1 



Nitrogen 1 



Hydrogen 1 



Carbonic oxide 7o0 



Carbonic acid 972 



Hydrochloric acid 1,005 



Kvcry gas in this tnble is perfectly transparent to light — that is to say, all 

 waves within the limits of the visible spectrum pass through it without obstruc- 

 tion ; but for the waves of slower period, emanating from our heated ])late of 

 copper, cnonnous differences of absorptive 2iower are manifested. These differ- 

 ences illustrate in the most unexpected manner the inlliience of chemical com- 

 bination. 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 

 5,460 units, olefiant gas 6,030 units, while sulphurous acid gas absorbs 6,480 

 units. Wluit becomes of the wave motion thus intercepted? It is apj)lied to 

 the heating of the absorbing gas. Through air, oxygen, hydrogen, and nitro- 

 gen, on the contrary, the waves of ether ])ass 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 avc have the same atoms in a state of chemical unicm that exis^ 

 uucoinbined in the atmosphere j but the absorption of the compound is 1,S0C 

 times that of the air. 



V. — FORMATION OF INVISIBLE FOCI. 



This extraordinary deportment of the elementary gases naturally ilirOcf^iJ 

 attention to elementary bodies in another state of aggregation. Some of Mel- 

 loni's results now attained a new signilicance } for this celebrated experimenter 

 had found crystals of the element sulpluu' to be highly pervious lo radiant lieat ; 

 he had also proved that lampblack and black glass (which owes its blackness 

 to the element carbon) w'ere to a consideral)le (,'xtent transparent to calorilic lays 

 of low refrangibility. These facts, harmonizing so strikingly witli the deport 

 ment of the simple gases, suggested fiu-ther in(p.iiry. Sulphur dissolved in bisul- 

 phide of carbon was fumid ahnost perj'ectly ti-ansparent. The dense and deeply 

 colored element bromine was examined, and found Ci)mpctent to cut off the light 

 of our most brilliant flames, while it transmitted t^ic invisible calorific rays with 

 extreme freedom. Iodine, the comi)anion element of bromine, was next thought 

 of, but it was found. im])raclicable to examine the substance in its usual solid 

 condition. It, however, dis.solves freely in bisulphide of carbon. There is no 

 chemical miion between the li(juid and the iodine; it is simply a case of solution, 

 in which the uncombined atoms ol" ttio element can act upon the radiant heat. 

 When }iermitted to do so, it was iunnd that a layer of disscdved iodine, sulliciently 

 opafpie to cut off the light of the mid-day sun, was almost absolutely trans})arent 

 to all invisible calorific rays. 



By i)rismat.ic analysis Sir William Ilersclad separatccl the luminous from the 

 non-luminous rays of the sun, and ho also sought to render the ol>scure rays visi- 

 ble by concentration. Intercepting the luminous portion of his sjjectruin ho 



