Absorption of Heat on the Formation of Dew. 115 



If atmospheric air was passed through the flask while the water 

 in it was heated to from 60° to 80°, the air thus saturated with 

 aqueous vapour produced a deflection which was irregular, and 

 sometimes increased to 20 millims. ; this was very gradual, while 

 the deflections which carbonic acid and coal-gas produced set 

 in at once, and rapidly increased to a maximum. It might be 

 thought that from the flask to the heated part of the brass tube 

 the vapours had so far been precipitated that there was here 

 little or no vapour. But, leaving out of sight that such a cool- 

 ing was impossible, because the short piece of tube from the 

 flask to the heated tube was always very warm, the great quan- 

 tity of aqueous vapour present in the radiating air could be 

 readily shown by holding a glass plate in it, on which water was 

 copiously deposited. 



When the water in the flask boiled so violently that there was 

 mist in the emergent air, the galvanometer produced a deflec- 

 tion of upwards of 100 divisions. The result was the same 

 when no air was passed through the flask, but the water in it 

 boiled so violently that the vapour issued from the heated tube, 

 in which case mist was always seen. If no mist was visible, 

 the galvanometer produced no greater deflection than twenty 

 divisions, however much vapour the air contained. From the 

 manner in which this comparatively small deflection occurred, 

 from its irregularity,, and the slow advance of the needle, one is 

 tempted to conclude that .this also depended on a formation of 

 mist produced at the boundary of the ascending current, but not 

 perceptible to the eye on account of its small quantity. A.s 

 soon as the greater deflection was produced, the mists could 

 each time be seen. They can be perceived with such certainty 

 that always, if one observer first noticed the mist, the other at 

 the telescope announced the motion of the galvanometer. 



I imagined that by using the very ingenious arrangement 

 which M. Toeppler has described in his paper, " Observations by 

 a New Optical Method," under the name Schlierenapparatus, 

 the mist could be still more readily perceived ; yet I soon found 

 that this artificial arrangement has here no advantage — the more 

 so, that a mistake as regards the occurrence of the mist was not 

 possible. 



Several of my friends also, who at different times were present 

 at the experiments, especially MM. Dove, Du Bois-Reymond, 

 Hofraann, Poggendorff, Quincke, and Riess, have convinced 

 themselves that air, when saturated at ordinary temperatures 

 with aqueous vapour, produced no greater deflection than about 

 3 millims., and, even when saturated at a higher temperature, 

 nothing greater than 20 millims. ; only when mist was visible was 

 the deflection as great as with carbonic acid — that is, more than 



12 



