138 ANNUAL OF SCIENTIFIC DISCOVERY. 



was placed under the jet at various points ; the cloud was cut sharply 

 off at that point, and when the flame was placed near the efflux orifice, 

 the cloud entirely disappeared. The heat of the lamp completely pre- 

 vented precipitation. 



This same vapor was condensed and congealed on the surface of a 

 vessel containing a freezing mixture, from which it was scraped in 

 quantities sufficient to form a small snow-ball. The beam of the elec- 

 tric lamp, moreover, was sent through a large receiver placed on an air- 

 pump. A single stroke of the pump caused the precipitation of the 

 aqueous vapor within, which became beautifully illuminated by the 

 beam ; while, upon a screen behind, a richly-colored halo, due to dif- 

 fraction by the little cloud within the receiver, flashed forth. 



The waves of heat speed from our earth, through our atmosphere, 

 toward space. These waves dasl^n their passage against the atoms 

 of oxygen and nitrogen, and against the molecutes of aqueous vapor. 

 Thinlv scattered as these latter are, we mig^ht naturally think meanlv 



V V t/ 



of them as barriers to the waves of heat. We might imagine that the 

 wide spaces between the vapor molecules would be an open door for 

 the passage of the undulations ; and that if those waves were at all in- 

 tercepted, it would be by the substances which form 99^ per cent, of 

 the whole atmosphere. Three or four years ago, however, it was 

 found by the speaker that this small modicum of aqueous vapor inter- 

 cepted fifteen times the quantity of heat stopped by the whole of the 

 air in which it was diffused. It was afterwards found that the dry air 

 then experimented with was not perfectly pure, and that the purer the 

 air became, the more it approached the character of a vacuum, and the 

 greater, by comparison, became the action of the aqueous vapor. 1 The 

 vapor was found to act with thirty, forty, fifty, sixty, seventy times the 

 energy of the air in which it was diffused ; and no doubt was enter- 

 tained that the aqueous vapor of the air which filled the Royal Institu- 

 tion Theatre, dining the delivery of the discourse, absorbed ninety or 

 one hundred times the quantity of radiant heat which was absorbed by 

 the main body of the air of the room. 



Looking at the single atoms, for every two hundred of oxygen and 



1 Melloni of Italy, who has been styled the " Newton of Heat," was the first to 

 apply the thermo electric pile to the investigation of dark, thermal radiations which 

 are emitted from all bodies below a red heat, by the use of plates, lenses, and 

 prisms of rock-salt (common salt in blocks), which is transparent to dark heat, 

 JVJelloni first engaged in these researches, and in fact founded this branch of science. 

 Prof. Tyndall, taking up the subject where Melloai left it, investigated the rela- 

 tions of radiant heat to gases and vapors in the following manner : He prepared a 

 long glass tube closed air-tight at its ends with plates of rook-salt, and which he 

 could empty and fill with various gases at pleasure. At one end he placed his 

 source of heat, a canister of hot water, and at. the other the sensitive face of a 

 thermo-electric pile. By exhausting the air and forming a vacuum, and then intro- 

 ducing various gaseous bodies, he determined how much dark heat passed through 

 and also the different absorbing or intercepting powers of the various substances 

 in the tube. It was found that the simple gases, oxygen, hydrogen, and nitrogen 

 arrested hardly a trace of the passing heat, acting toward it a.s practical vacuum. 

 On the contrary, other equally transparent gases, as ammonia, carburetted hydro- 

 gen, etc., stopped enormous numbers of the dark rays ; in fact, were almost opaque 

 to them. The small trace of ammonia, exhaled into an apartment by opening a 

 lady's smelling-bottle arrested many times more of the dark heat rays than the ni- 

 trogen and oxygen gases which form the body of the atmosphere. Professor 

 Tyndall found that perfectly pure air stopped an exceedingly minute portion of the 

 heat, which he assumed as the unit for comparison of other bodies, and upon in- 

 vestigation it proved that the small amount of aqueous vapor contained in the air 

 struck down sixty or seventy times as much heat as the gases of the air itself. 



