500 Prof. Tyndall on the Action of Free Molecules on 



sulphuric ether. "When a large evaporating-surface is ex- 

 posed, there is therefore no difficulty in obtaining, from this 

 liquid, vapour of a pressure of 6*6 inches of mercury. This, 

 in a tube 38 inches long, would, if squeezed to liquefaction, 

 produce a layer 1 millimetre thick. Vapour absorption and 

 liquid absorption being measured in succession, this is the 

 behaviour of the hydride of amyl : — 



Absorption by vapour . . . . 51 per cent. 

 Absorption by liquid .... 51 „ 



the two absorptions being absolutely identical *. 



Combining this section and the last, their joint results may 

 be thus summed up. Beginning with a column of sulphuric- 

 ether vapour 38 inches long at 7*2 inches pressure, or with a 

 column of hydride-of-amyl vapour 38 inches long and at 6'6 

 inches pressure, and gradually shortening the column without 

 altering the quantity, the vapour would gradually augment in 

 density and pass wholly, when reduced to a thickness of 1 

 millimetre, into the liquid state of aggregation. Suppose a 

 beam of heat of constant value, after passing through the 

 vapour, to impinge upon a thermopile and to produce a 

 definite galvanometric deflection; this deflection would remain 

 absolutely fixed during all the charges of density and aggre- 

 gation which we have supposed the vapour to undergo. In 

 other words — as regards the absorption of radiant heat, the 

 vapour would pass, without breach of continuity, through all 

 its stages of condensation into the liquid form of matter. 



A general law of molecular physics is, I apprehend, here 

 illustrated. 



§ 8. Rhythmic Absorption of Radiant Heat by Gases and 

 Vapours. 



Conclusive as the foregoing experimental argument must 

 appear as regards the action of free molecules upon radiant 

 heat, I am nevertheless glad to supplement it by another of a 

 totally different character. On the 29th of November, 1880, 

 I had the pleasure of witnessing, in the laboratory of the 



* "When the rock-salt cell was empty, reflection of course occurred at 

 its two interior surfaces. A perfectly diatherruanous liquid, with the 

 refractive index of rock-salt, would annul this reflection. And though 

 the liquids actually employed had a smaller refractive index than rock- 

 salt, and though they were far from, being perfectly diathermanous, their 

 introduction into the cell must nevertheless have diminished the reflec- 

 tion, and thus added to the transmitted heat. This addition, having been 

 determined by calculation, was sensibly neutralized by the introduction 

 of washers of thin note-paper, which slightly augmented tkethicknefB of 

 the liquid stratum traversed by the caloriiic rays, 



