340 Prof. Tyndall on the Absorption and 



The paradox already referred to is here solved, and the expla- 

 nation given of the extraordinary effect observed in the case of 

 the alcohol and ether vapours when dry air entered the experi- 

 mental tube. Dynamic radiation, moreover, and dynamic ab- 

 sorption go hand in hand ; and if we compare both with Table 

 III., we shall find the order of the substances precisely the same, 

 although one set of results are obtained with a source of heat ex- 

 ternal to the gaseous body, and the other with a source of heat 

 and cold within the body itself. Had I sufficient time at my 

 disposal, I could develope this subject with advantage. The 

 results just recorded constitute my first regular series of experi- 

 ments ; and, no doubt, augmented experience will enable me to 

 attain more perfect results. 



I could not well obtain half an inch of my most energetically 

 acting vapour, namely, boracic ether; but one-tenth of an inch 

 admitted into the tube and dynamically heated and chilled, 



gave — 



Radiation. Absorption. 



56° 28° 



Seeing the astonishing energy with which some of these 

 vapours absorb and radiate heat, it may be asked how far the 

 quantity of vapour may be reduced before its action becomes 

 insensible. At present I will not venture to answer this question 

 fully; certainly we should be dealing at least with millionths of 

 our smallest weights. But I will here lay before the Society an 

 account of one experiment, the result of which can hardly fail to 

 excite astonishment. The experimental tube being exhausted, 

 one-tenth of an inch of boracic-ether vapour was admitted into 

 it : the barometer stood at 30 inches at the time ; hence the ten- 

 sion of the vapour within the tube was 3-3^ th °^ an atmosphere. 



Dynamically heated by dry air, the radiation of this vapour 

 produced a deflection of 56°. 



The tube was then exhausted to 0*2 of an inch, and the quan- 

 tity of vapour reduced thereby to j-jo^ part of its first amount ; 

 the needle was allowed to come to zero, and the residue of vapour 

 was dynamically heated as before : its radiation produced a de- 

 flection of 42°. 



The pump was again worked till a vacuum of 0*2 of an inch 

 was obtained, this residue containing of course yj-Q-th of the 

 quantity of ether present in the last On dynamically heating 

 this residue, its radiation produced a deflection of 20°*. 



Two additional exhaustions, succeeded by dynamic heating, 

 gave the deflections 14° and 10° respectively. 



* This is less than the truth — my assistant having executed three or 

 four strokes of the pump inadvertently while the dry air was not shut off, 

 removing thereby a considerable proportion of the vapour which ought to 

 be present at this stage of the experiment. 



