RADIATION. 307 



act of .ihsorption dcponils upon the individual molocnlo, which equallv asserts 

 its power in tlie liquid and the p;aseous state. "\V(^ may asstuvdly conclude iVoni 

 the above table tliat the position of a vapor is determined by that of its liquid. 

 Now, at the very foot of the list of liquids stands water, signaliziui^ itself ab(»vc' 

 all others by its enormous power of abs<)r[)tion ; and from this fact, even if ii.» 

 din'ct experiment on the vapor of water had ever been made, we should be enti- 

 tled to rank tliat vapor as the most powerful absorber of radiant heat hitherto 

 discovered. It has been proved by experiment that a .shell of air two inclics in 

 thiclvuess surrounding our ])lanet,:wid satiu'ated witli the vapor of sul])huric cllicr, 

 would intercept 35 per cent, of the earth's radiation; and thou<Th the quantity (d 

 aqueous vapor necessai'V to saturate air is much less than the anuMuit of sulphuric 

 etlier vapor which it can snst.ain, it is still extremely jtrobablc that the estimate 

 already made of the action of atmospheric vapor within 10 feet of the earth's 

 surface is altogether under the mark, and that we are indebtccl to this wonderful 

 substance, to an extent not accurately determined, but certainly far beyond what 

 has hitherto been imagined, for the tem])erature now existing at the surface of the 

 globe. 



XIV. EECIPKOCITT OF RADIATIOX AND ABSOIU'TIOX. 



Throughout tlie reflections which have hitherto occu|iied us the image before 

 the mind has been that of a radiant source generating calorific waves, which, on 

 passing among the sc^ittered molecules of a gas or vapor, were intercepted by 

 those molecules in various degrees. In all cases it was the transference of 

 motion from the ether to the comparatively (juiescent molecules of the gas or 

 vapor. We have now to change the form of our conception, and to figure these 

 molecules not as absorbers but as radiators — not as the recipients l)ut as the 

 originators of wave motion ; that is to say, we must figure them vibrating and 

 generating in the surrounding ether unduhitions which speed through it with the 

 velocity of light. Oiu* object now is to inquire whether tlie act of chemical com- 

 bination, which proves so potent as regards the phenomena of absorption, does 

 not also manifest its power in the phenomena of radiation. For the examination 

 of this question it is necessary, in the first place, to heat our g;uses and vaj)ors t<» 

 the same temperature, and then examine their power of discharging the motion 

 thus imparted to them upon the ether in which they swing. 



A heated copper ball was placed al)ove a ring gas-burner possessing a great 

 number of small apertures, the burner being connected by a tube with vess(ds 

 ctmtaining the various gases to be examined, liy a gentle ])ressure tlu) gases 

 were forced through the orifices of the burner against tlie copper ball, where 

 each of them, b(;ing heated, rose in an ascending column. A therjiio-electric 

 jnle, entirely screened olf from the hot ball, was exposed to the radiation of the 

 warm gas, and the deflection of a magnetic needle connected with the pile 

 dechired the energy of th(^ radiation. 



Bv this mode <if experiment it was prov(;d that the self-same mtdecular arrang(>i- 

 ment which renders a gas a powerful absorber renders it in the same degree a 

 powerful radiator — that the atom or molecide which is (■onq)etent to intercept the 

 calorific waves is, in the same degree, conqtetent to generate them. Thus, while 

 the atoms of elementary gjuses proved th(;mselves unable to emit any sensible 

 amount (jf radiant heat, the molecules of compound gases were shown to be capa- 

 ble oi' j)owerfully disturbing the surrounding ether. 13y special modes of experi- 

 ment the same was ])roved to hold goo<l for the vajiors of volatile li(piids, tlie 

 r.'idiative pow<H' of every vapor being found proportional to its absorptive power. 

 These experiments were based uj)(mi the fact that atoms, such, for exanqile, as 

 those of air, which glide through the ether without sensible resistance, cannot 

 thus glide among the molecules of another gas. When mixed with such mole- 

 cules, the heated atoms comnnniicate their UKition to the m<decules by direct col- 



