244 Prof. Magnus on the Condensation of Vapours 



the temperature of the pile and of the air was the same. This 

 experiment was therefore always available to ascertain whether 

 this equality of temperature was present or not. If the air of 

 the room, previously very far off saturation, were then saturated 

 with moisture, so great an elevation of temperature resulted, that 

 (the pile not being covered with a plate) the deflection of the mirror 

 could no longer be observed, since the image of the scale vanished 

 out of the field of the telescope. To an equal extent, but in an 

 opposite direction, the deflection took place when the air of the 

 room, previously moist, was dried and then admitted to the pile. 



When the pile was shielded by a plate, the deflection of the 

 galvanometer was less ; and on employing different plates, the 

 deflection was of various extent, according to the nature of the 

 plates, their thickness, and their extent. But it was visible with 

 all the plates which I could try, both with rough and with smooth 

 surfaces, and even when they were coated with varnish. In the 

 instance of a brass plate of 2*5 millims. in thickness, the galva- 

 nometer gave a stroke of more than fifty divisions of the scale 

 when moist air was blown against the plate. On continuing 

 uninterruptedly to blow moist or dry air, the galvanometer very 

 gradually turned back to its position indicating equilibrium. 

 The temperature of the room being low, the employment of moist 

 air occasioned only a gradual elevation of the temperature of the 

 plate, doubtless because the air contained only little mois- 

 ture. If, however, after a certain deflection of the galvanometer 

 had been reached dry air were blown in, then the galvanometer 

 struck quickly towards the opposite side, and went, in conse- 

 quence of the sudden cooling, far away beyond equilibrium. 



Glass plates acted similarly to metallic plates. Also in this 

 case the deflection varied with their thickness and extent. The 

 deflection was particularly marked when a thin glass plate was em- 

 ployed, such as is used for the polarization of light, or for the 

 covering of microscopical objects. Plates of quartz, of gypsum, 

 of mica, of rock-salt, of alum, showed the same phenomenon. 



Plates of wood, paste, and different other organic bodies 

 behaved in the same way, as likewise caoutchouc, both vulcanized 

 and non -vulcanized, gutta percha, leather both greased and un- 

 greased, ivory, &c. There was a deflection of the galvanometer 

 of 100 divisions of the scale and more when moist air was blown 

 against plates of parafiin, stearic acid, and wax, of about 2*5 mil- 

 lims. in thickness. 



By employing leather which was not greased, pasteboard, thin 

 wood, ivory, gutta percha, and some other substances, the deflec- 

 tion of the galvanometer was at least as great (and sometimes 

 even greater) as in the instance in which dry or moist air was 

 blown immediately against the pile. This arises undoubtedly 



