1879.] Buff's Experiments on the Diathermancy of Air. 15 



by dry and pellucid plates of rock-salt. The vacuous chamber was 

 clasped at one place by a water jacket, which cut off all conduction 

 from the source. At some distance beyond the end of the tube stood 

 tke thermo-pile, furnished with a double cone, with a view to the 

 application of the principle of compensation. The experimenter who 

 does not shrink from the discipline necessary to master it will find 

 this method powerful, exact, and under the most complete control. 

 By it was determined, for the first time, in 1859, the action of a con- 

 siderable number of gases and vapours, one result of the inquiry being 

 that, while some transparent gases were more impervious than many 

 solids, the dry atmospheric air enveloping our earth was a practical 

 vacuum to the rays of heat. 



From this result the experiments of the late Professor Magnus led 

 him to dissent. We once discussed our differences viva voce ; he urging 

 against me an objection which has been revived by Professor Buff. 

 I had, he said, sent my heat through a vacuous chamber into my 

 experimental tube ; but between that tube and my pile there existed 

 a space of air, in which, he asserted, the absorption which I had failed 

 to detect in the tube itself took place. He mentioned a series of 

 -experiments which he was then on the point of publishing, and which, 

 he said, proved, not only that the absorption of radiant heat by air 

 was considerable, but also that a layer of air 12 inches thick sufficed 

 to intercept all the rays that air was capable of intercepting. A pub- 

 lished account of these results I have never been able to find, but they 

 must have closely resembled those now under discussion. 



I asked Professor Magnus whether he thought a stratum of air 

 -^th of an inch in thickness would exert any sensible absorptive 

 -action. He promptly replied in the negative. I therefore made the 

 following experiment, which has been overlooked by Professor Buff. 

 Placing the anterior cone of my pile within the experimental tube, I 

 was able to bring its naked face within -^th of an inch of my terminal 

 plate of rock-salt. There was not the slightest alteration of the pre- 

 viously obtained result. Dry air, as before, behaved like a vacuum. 



Pig. 1 is a rough vertical section of one of the pieces of apparatus 

 recently arranged with a view of testing Professor Buff's conclusions. 

 P and P are two chambers formed of brass tubing 2J inches in diameter. 

 The chamber F is soldered to the cube C, one face of which consti- 

 tutes the source of heat. W is a jacket in which water of the 

 temperature of the surrounding atmosphere constantly circulates, 

 preventing the heat conducted from C from passing further. The 

 chamber F is separated from the chamber P by an intermediate 

 chamber i, bounded by the rock salt plates r, r'. Within the chamber P 

 is the thermo-pile, from which wires pass through the tube to a sensi- 

 tive galvanometer. S is a screen, and C a compensating cube. The 



