27.2 Prof. Tyndall on the Absorption and 



But in these experiments my first source of heat stood at some 

 distance from the anterior end of the tube, and the heat, pre- 

 viously to entering the latter, had to cross a space of air which 

 was not the subject of examination. This air-space I wished to 

 abolish, so as to allow the calorific rays to enter the gas with all 

 the qualities which they possessed at the moment of emission. 

 I first thought of soldering the end of the experimental tube 

 direct to the radiating surface, thus allowing the air to come 

 into direct contact with the source. But it immediately occurred 

 to me that the introduction of cool air into the tube would lower 

 the temperature of the source, and that I could never know how 

 far the indication of my galvanometer under such circumstances 

 could be regarded as a true effect of absorption ; hence I aban- 

 doned the idea of bringing the gases into contact with the radia- 

 ting surface. 



Instead of this arrangement an independent tube, 8 inches 

 long, and of the same diameter as the experimental tube, was 

 soldered on to the radiating plate. By means of a screw joint, 

 the free end of this tube was connected air-tight with the expe- 

 rimental tube. Thus a chamber, from which the air could be 

 removed, was introduced between the first plate of salt and the 

 radiating surface. Two objects were thus secured : first, my 

 source of heat was withdrawn from the action of irregular cur- 

 rents of air ; and secondly, the radiant heat entered the tube 

 unchanged in quality save the infinitesimal change due to its 

 passage through the diathermic salt. 



T© save the trouble and expense of a new Plate, I will ask 

 permission to refer in this memoir to the Plate (PI. III. vol. xxii.) 

 made use of in my last ; a verbal reference will in most cases 

 be sufficient to indicate the changes recently introduced. S S f 

 it will be remembered, represented the experimental tube, which 

 was then made of brass polished within. Such a tube could not 

 be used for any gases or vapours capable of attacking brass ; and 

 though I combated this difficulty, to some extent, by blackening 

 the tube within, I could never feel at ease regarding the action 

 of the gases upon the blackening substance. Many gases, more- 

 over, present great difficulties on account of their affinity for 

 atmospheric moisture. Hydrobromic and hydrochloric acid, for 

 example, form dense fumes in the air; and however carefully they 

 might have been dried, I should have been reluctant to base any 

 inference on their deportment without actually having them under 

 my eyes during experiment. 



The brass tube, then, which stretched from S to S' in the 

 figure is now replaced by one of glass, 2 feet 9 inches long, and 

 2*4 inches in diameter. The source of heat in my last-published 

 inquiry was the cube of hot water C ; but glass being far inferior 



