202 Prof. Tyndall on Radiation 



receives the name of Radiant Heat; and if competent to excite 

 the nerves of vision, we call it Light. 



Aqueous vapour was defined to be an invisible gas. Vapour 

 was permitted to issue horizontally with considerable force from 

 a tube connected with a small boiler. The track of the cloud 

 of condensed steam was vividly illuminated by the electric light. 

 What was seen, however, was not vapour, but vapour condensed 

 to water. Beyond the visible end of the jet the cloud resolved 

 itself into true vapour. A lamp 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 vapour was condensed and 

 congealed on the surface of a vessel containing a freezing-mix- 

 ture, from which it was scraped in quantities sufficient to form 

 a small snowball. The beam of the electric 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 

 vapour within, which became beautifully illuminated by the 

 beam ; while, upon a screen behind, a richly coloured halo, due 

 to diffraction by the little cloud within the receiver, flashed forth. 



The waves of heat speed from our earth through our atmo- 

 sphere towards space. These waves dash in their passage 

 against the atoms of oxygen and nitrogen, and against the 

 molecules of aqueous vapour. Thinly scattered as these latter 

 are, we might naturally think meanly of them as barriers to the 

 waves of heat. We might imagine that the wide spaces between 

 the vapour-molecules would be an open door for the passage of 

 the undulations, and that, if those waves were at all intercepted, 

 it would be by the substances which form 99-J- per cent, of the 

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

 found by the speaker that this small modicum of aqueous vapour 

 intercepted 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 vapour. The vapour was 

 found to act with 30, 40, 50, 60, 70 times the energy of the 

 air in which it was diffused; and no doubt was entertained that 

 the aqueous vapour of the air which filled the Royal Institution 

 theatre during the delivery of the discourse, absorbed 90 or 100 

 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 200 of oxygen and 

 nitrogen there is about 1 of aqueous vapour. This 1, then, is 



