AND ETHER WAVES. 91 



of men across Pall Mall. Let us therefore push matters 

 to extremes, and continue the condensation till the 

 vapour has been squeezed into a liquid. To the pure 

 change of density we shall then have added the change 

 in the state of aggregation. The experiments here are 

 more easily described than executed; nevertheless, by 

 sufficient training, scrupulous accuracy, and minute 

 attention to details, success may be ensured. Knowing 

 the respective specific gravities, it is easy, by calcula- 

 tion, to determine the condensation requisite to re- 

 duce a column of vapour of definite density and length 

 to a layer of liquid of definite thickness. Let the 

 vapour, for example, be that of sulphuric ether, and let 

 it be introduced into our 38-inch tube till a pressure of 

 7.2 inches of mercury is obtained. Or let it be hydride 

 of amyl of the same length, at a pressure of 6.6 

 inches. Supposing the column to shorten, the vapour 

 would become proportionally denser, and would, in 

 each case, end in -the production of a layer of liquid 

 exactly 1 millimeter in thickness.* Conversely, a layer 

 of liquid ether, or of hydride of amyl, of this thickness, 

 were its molecules freed from the thrall of cohesion, 

 would form a column of vapour 38 inches long, at a 

 pressure of 7.2 inches in the one case, and of 6.6 inches 

 in the other. In passing through the liquid layer, a 

 beam of heat encounters the same number of molecules 

 as in pasing through the vapour layer; and our prob- 

 lem is to decide, by experiment, whether, in both cases, 

 the molecule is not the dominant factor, or whether its 

 power is augmented, diminished, or otherwise over-rid- 

 den by the state of aggregation. 



Using the sources of heat before-mentioned, and em- 

 ploying diathermanous lenses, or silvered mirrors, to 

 render the rays from those sources parallel, the absorp- 

 * The millimeter is j^th of an inch. 



