Vapour in Experiments on the Absorption of Heat' 423 



end nearest the pile, whether the narrowing be effected by in- 

 troducing a ring or by a diaphragm fastened to the tube. The 

 smaller the aperture of the latter, the greater the quantity of 

 air which reaches the pile. 



If the diaphragm is at the end more distant from the pile, 

 less air reaches the pile than if it is fastened on the one nearest ; 

 and if there are diaphragms at both ends, no more air reaches 

 the pile than if only one is introduced at the end nearest the 

 pile. 



This may seem surprising; but it must be remembered that 

 by the presence of diaphragms at the ends of the tubes the dif- 

 fusion of air in the interior is not altered, as long as the aper- 

 ture is large enough to allow the air to emerge as fast as it 

 enters. The narrower this aperture the greater is the velocity 

 with which the air emerges, and the further does it continue its 

 path. 



With tubes of 60 centims. diameter, provided with a dia- 

 phragm at the end turned towards the pile, air reached the pile 

 even when the current was very feeble, or when the conducting 

 tubes were half closed, provided the diameter of the diaphragm 

 was but little less than the cone of the pile, the aperture of which 

 was 56 millims. in diameter. For when moist air was blown 

 into the tube thus arranged without using a source of heat, 

 heating ensued — not regularly, but almost every time. 



An attempt was made to render visible the motion of the air 

 in the tubes. For this purpose the air passed, first, through a 

 vessel which contained concentrated hydrochloric acid, and then 

 through another with ammonia, and, thus charged with a cloud 

 of sal-ammoniac, was forced into a glass tube of the same di- 

 mensions as the brass one, and under the same pressure. The 

 cloud issued through the narrow apertures of the diaphragms 

 much further than from the wider one, by which the fact of the 

 action mentioned acquires a confirmation. But the experiments 

 showed also that the motion of air in these tubes is very easily 

 disturbed, and that the entire method is quite unfitted for mea- 

 surements. 



I regret that in my earlier experiments I did not use tubes 

 with a greater diameter than the cone of the pile ; for I should 

 then probably have obtained results analogous to those of 

 Messrs. Tyndall and Wild. But as wider tubes are not so com- 

 pletely filled with air as narrower ones, and as no particularly 

 wide tubes are necessary for observing absorption, I did not see 

 any necessity for extending my researches to such tubes. 



In conclusion I must refer again to Professor Wild's paper. 

 Experiments have been made by him by a method which 

 he designates by my name. In spite of manifold variations 



