242 Prof. Magnus on the Condensation of Vapours 



Since the metals of which the thermo-piles are usually con- 

 structed possess, from their crystalline structure, a kind of porous 

 nature, it was conceivable that it was in consequence of this that 

 the pile exhibited the phenomena in question. But when thin 

 platinum-foil was laid upon the pile and alternately a current of 

 dry and of moist air directed against it, the same depression and 

 elevation of temperature, only less pronounced, took place in 

 presence of the covering of foil as had been noticed without it. 

 Of necessity, then, the elevation of temperature due to absorption 

 on one side of the foil must have been sufficient to occasion an 

 elevation of temperature on the other side sufficient to affect the 

 pile to an appreciable extent. 



The known property of platinum, of causing combination 

 between gases, could not have occasioned this phenomenon, for it 

 soon became apparent that other metals, when they were sub- 

 stituted for platinum, occasioned a similar elevation of tempe- 

 rature. 



In order to ensure that the pile did not come into direct rela- 

 tion with the air, and that the rise of temperature was due to 

 the plate, the following apparatus was employed. 



The plate pp (Plate III. fig. 1), which was to be experimented 

 upon, was laid upon the upper border of an upright glass tube, 

 RR, of 150 millims. in length and of 35 millims. in diameter, 

 open at both ends, and provided above with a well-ground rim. 

 Upon this plate was placed the thermo-pile s s, which was pro- 

 vided at its upper part with a brass cap, and was pressed against 

 the plate pp by means of a spiral spring x x. The pile, with 

 the bow b b, which served to allow of the fastening of the spiral 

 spring, was covered with a glass shade, N N, which was closed 

 below by means of a glass plate M M, through which the glass 

 tube II R passed. The plates pp to be operated upon were 

 quite flat. When they could not be got large enough to close 

 the tube R R, then a glass plate with an opening in the middle, 

 somewhat larger than the diameter of the pile, was placed imme- 

 diately in cod tact with the tube. The plate to be used was then 

 applied to this glass plate, and the pile pressed against it by 

 means of the spiral x x. By means of a bellows the air was 

 forced into the tube R R through a caoutchouc tube fg, on the 

 end of which there was a glass tube g g. This glass tube g g was 

 brought vertically into the middle of the tube R R, terminating 

 at a distance of 40 millims. below the plate pp to be experi- 

 mented upon, and so the air streamed against the plate from this 

 distance. In order at will to have either dry or moist air, tubes 

 provided with stopcocks, and containing in one case chloride of 

 calcium and in the other fragments of glass moistened with 

 water, were inserted between the bellows and the caoutchouc 



