Heat through moist Air. 251 



sphere with a solution of salt, be placed in dry air, the water eva- 

 porates and the salt becomes again dry. In the experiments I 

 made, it was merely necessary to expose the plates in the labo- 

 ratory to obtain them quite dry in a few hours. 



Melloni* found that a layer of pure water of a millimetre 

 in thickness transmits no heat which comes from an obscure 

 source of heat. Of the rays of ignited platinum, 5*7 per cent, 

 are transmitted. In Melloni's experiments f, a saturated solution 

 of rock salt transmitted -^ more of the rays from an Argand 

 lamp, than a layer of water of the same thickness, and in Dr. 

 Franz's experiment ^ more. As far as I know, no experiments 

 have been published as to the proportion of rays transmitted 

 through a very thin layer of solution of rock salt ; but the 

 quantity which is transmitted must in any case be excessively 

 small. Hence even the thinnest layer of solution on the plate 

 hinders the passage of heat. 



In order to investigate how far this is the case, the following 

 experiment was made : — A tube of strong glass, a metre in length, 

 was closed at both ends by plates of English rock salt a milli- 

 metre thick. It was first rilled with dry air, by sending through 

 it, by means of an aspirator, such a quantity of air, which had 

 been dried by passing through several chloride-of-calcium tubes, 

 as to make it quite certain that all the original air had been dis- 

 placed. Thereupon the deflection of the galvanometer was observed, 

 which was produced when the rays from a flask blackened on the 

 outside and filled with water kept boiling bypassing a current of 

 steam through it reached the thermo-pile after passing through 

 the tube full of dry air. Then, while the rest of the experiment 

 was unchanged, air, which had previously passed through a tube 

 filled with moistened pumice, was sent through the same tube. 

 So soon as only a small quantity of this air filled the tube with 

 the plates of rock salt, the quantity of the heat which reached 

 the thermo-pile, after traversing this air, decreased. If, then, dry 

 air was again transmitted through the tube, the deflection again 

 increased and finally attained its original value. It is perhaps 

 superfluous again to remark that, when the tube was closed with 

 glass plates instead of rock-salt plates, nothing was observed of 

 such a difference in the transmission of the thermal rays. By a 

 continued transmission of moist air through the tube with the 

 rock-salt plates, the quantity of heat which traversed it could 

 easily be reduced to J. In that case the plates, when removed 

 from the tube, were found to be covered on the inside with 

 moisture. A decrease such as Mr. Tyndall mentions J, to T \y 

 or to^V, has not, could not be attained in these experiments, not 



* La Thermochrose, 207. t Ibid. 165. 



% Phil. Mag. vol. xxii p. 377. 



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