448 Prof. TyndalPs Contributions to Molecular Physics, 



Had it been desirable to push these measurements to the 

 utmost limit of accuracy, I should have repeated each experi- 

 ment, and taken the mean of the determinations. But consi- 

 dering the way in which the different thicknesses check each 

 other, an inspection of the Table must produce the conviction 

 that the results express, within small limits of error, the action 

 of the bodies mentioned. 



§ III. Absorption of radiant heat of the same quality by the 

 vapours of those liquids at a common tension. 



As liquids, then, those bodies are shown to possess very 

 different capacities of intercepting the heat emitted by our radia- 

 ting source ; and we have next to inquire whether these differ- 

 ences continue after the molecules have been released from the 

 bond of cohesion. We must, of course, test the vapours by waves 

 of the same period as those applied to the liquids ; and this our 

 mode of experiment renders easy of accomplishment. The heat 

 generated in a wire by a current of a given strength being inva- 

 riable, it was only necessary, by means of the tangent compass 

 and rheocord, to keep the current constant from day to day in 

 order to obtain, both as regards quantity and quality, an invari- 

 able source of heat. 



The liquids from which the vapours were derived were placed 

 in a small long ilask, a separate flask being devoted to each. 

 The air above the liquid and within it being first carefully 

 removed by an air-pump, the flask was attached to the experi- 

 mental tube in which the vapours were to be examined. This 

 tube was of brass, 49*6 inches long, and 2*4 inches in diameter, 

 its two ends being stopped by plates of rock-salt. Its interior 

 surface was polished. At the commencement of each experiment, 

 the tube having been thoroughly cleansed and exhausted, the 

 needle stood at zero*. The cock of the flask containing the 

 volatile liquid was then carefully turned on, and the vapour 

 allowed slowly to enter the experimental tube. The barometer 

 attached to the tube was finely graduated, and the descent of the 

 mercurial column was observed through a magnifying lens. 

 When a pressure of 05 of an inch was obtained, the vapour was 

 cut off and the permanent deflection of the needle noted. Know- 

 ing the total heat, the absorption in LOOths of the entire radia- 

 tion could be at once deduced from the deflection. The follow- 

 ing Table contains the results of the first series of experiments 

 made with the platinum spiral as source. 



* It is hardly necessary to remark that the principle of compensation 

 described in my former memoirs was employed here also. 



