10 



Prof. J. TyndaU on 



[Dec. 18, 



It is hardly possible that such general reasoning, as has been above 

 applied to the two problems of the present paper, could ever have 

 led to a discovery of the laws of change of the system. This kind of 

 consideration is, however, of some interest as throwing light on the 

 definite results already attained by the accurate methods of analysis. 



II. " On Buff's Experiments on the Diathermancy of Air." By 

 Johx Tyxdall, F.R.S. Received December 10, 1879. 



Two years ago, Dr. Hofmann drew my attention to a forthcoming 

 paper by Professor Buff, to which he obviously and Daturally attached 

 considerable importance. The paper appeared in the " Philosophical 

 Magazine " for December, 1877. Being much occupied at the time 

 with other matters, I merely glanced at its conclusions, and then laid 

 it aside until I should be able to read it carefully, and, if necessary, to 

 examine it experimentally. Last summer, I, for the first time, read 

 the paper through, and I have recently, more than once, repeated its 

 perusal — reflecting on its methods and conclusions, to the best of my 

 ability, wherever they appeared dark to me. 



The principal result of the paper is, that a stratum of dry air y 

 45 millims. thick, " absorbs from 50 to 60 per cent, of the rays of 

 heat which it receives from a source heated to the temperature of 

 boiling water." The experiments whereby I sought to show the 

 absorption of radiant heat, by a stratum of dry air more than thirty 

 times the thickness of that employed by Professor Buff, to be sensibly 

 nil, are, at the same time, pronounced "unreliable." 



I once ventured te express the opinion that 10 per cent, of the 

 radiation from the earth is absorbed by the aqueous vapour contained 

 in the first 10 feet of air. The late Professor Magnus urged against 

 me at the time, that were so much heat lodged in so thin a stratum, 

 the deposition of dew would be impossible. Urged against the con- 

 clusion that not 10 per cent., but 60 per cent, of the earth's radiation is 

 absorbed, not within 120 inches, but within 2 inches of the earth's 

 surface, the argument of Professor Magnus would have serious force. 

 Under the covering assigned to it by Professor Buff, our planet ought 

 never to suffer from rapid nocturnal chill. 



Professor Buff's intention, at starting, was to investigate the con- 

 duction of heat by gases, and he employed for this purpose an 

 apparatus similar to that of Professor Magnus, the dangers attending 

 the use of which I have frequently pointed out. A glass cylinder, 

 with its lower edge ground level, was mounted on the plate of an air- 

 pump. Cemented on to the open top of the cylinder was a brass 

 vessel with a polished horizontal bottom, which was heated by the 



