Prof. Tyndall on some Physical Properties of Ice. 341 

 communicated to the ice during the time occupied in melting a 

 quantity of the latter equal in volume to the bubble, which time 

 is stated to be brief; that is to say, the quantity of heat supposed 

 to be absorbed by the air would, if it had not been communicated 

 to the ice, have been sufficient to raise the bubble itself to a 

 temperature 160 times that of fused cast iron. Had air this 

 power of absorption, it might be attended with inconvenient 

 consequences to the denizens of the earth ; for we should dwell 

 at the bottom of an atmospheric ocean, the upper strata of which 

 would effectually arrest all calorific radiation. 



28. It is established by the experiments of Delaroche and 

 Melloni*, that a calorific beam, emerging from any medium 

 which it has traversed for any distance, possesses, in an exalted 

 degree, the power of passing through an additional length of the 

 same substance. Absorption takes place, for the most part, in 

 the portion of the medium first traversed by the rays. In the 

 case of a plate of glass, for example, 17| per cent, of the heat 

 proceeding from a lamp is absorbed in the first fifth of a milli- 

 metre, whereas, after the rays have passed through 6 millimetres 

 of the substance, an additional distance of 2 millimetres absorbs 

 less than 2 per cent, of the rays thus transmitted. Supposing 

 the rays to have passed through a plate 25 millimetres, or an inch 

 in thickness, there is no doubt that the heat emerging from such 

 a plate would pass through a second layer of glass, 1 milli- 

 metre thick, without suffering any measureable absorption. For 

 an incomparably stronger reason, the quantity of solar heat ab- 

 sorbed by a bubble of air at the earth's surface, after the 

 rays have traversed the whole thickness of our atmosphere, and 

 been sifted in their passage through it, must be wholly in- 

 appreciable. 



29. To the sifting power of the atmosphere we must add, in 

 the case of the glacier, the absorptive power of the ice. Some 

 notion of this power, as compared with that of air, may be 

 gathered from the following facts : — As regards the variation of 

 the intensity of radiant heat with the distance, the law of inverse 

 squares is capable of the strictest experimental verification in air, 

 even when the source of heat is far below 212° F. This implies 

 that the absorption in the space of air through which the heat 

 passes is too small to disturb the harmony of the la\y. Now a 

 ])late of ice, one-tenth of an inch thick, is absolutely impervious 

 to heat emanating from a source, not only of 212°, but of 752° F. ; 

 and is capable, moreover, of absorbing 99^ per cent, of the 

 calorific rays emitted by an incandescent platinum wiref- [La 

 Thermochrose, p. 164.)» 



* ha Thermochrose , p. 202. 



t I can hardly foibeai- drawiiiff attention lieie to the leniiukiible influence 



