Radiation of Heat by Gaseous Matter. 431 



It will be remembered that Professor Magnus obtained a 

 greater absorption with hydrogen than with either oxygen or 

 air. This result is perfectly explained by reference to the quicker 

 convection of this gas. I operated with hydrogen as I did with 

 air, first satisfying myself that a column of it 33 inches long 

 exercised an absorption less than unity : in fact it could not be 

 measured. The same hydrogen introduced into the first chamber, 

 and allowed to remain there for two minutes, caused a with- 

 drawal of heat from the source equivalent to an absorption of 65. 

 Now the absorption of air in Professor Magnus's experiments is 

 to that of hydrogen as 



1112: 1421, 

 or as 



50 : 64, 



while my results of convection are as 



50 : 65. 



The coincidence is so perfect that I am disposed to regard it as 

 in part accidental. 



Substantially the same remarks apply to the experiments with 

 the glass tube stopped with plates of glass 4 millimetres thick. 

 According to Melloni, 61 per cent, of the rays of a Locatelli 

 lamp are absorbed by a plate of glass only 2*6 millimetres thick. 

 Professor Magnus surrounded his flame by a glass cylinder ; and 

 this, it may be urged, partially sifted the heat of the lamp before 

 it reached the end of the tube. But in so doing the glass 

 cylinder itself must become intensely heated ; and to the heat of 

 the cylinder the glass ends of the tube would be opake • they 

 would absorb it all. Cold air admitted into such a tube is exactly 

 similar to cold air let into my front chamber ; it chills what is in 

 part the source of heat, and maintains that chill by convection. 

 The heat applied may, in fact, be thus analysed. 1 . We have a 

 portion, almost wholly luminous, which went*through the tube 

 direct to the pile ; 2, a portion arrested by the first glass plate ; 

 3, a smaller portion arrested by the second glass plate ; 4, we 

 have the heat radiated by the first glass plate towards the second, 

 and wholly absorbed by the latter; 5, we have the heat radiated 

 by this latter against the pile. This analysis will probably 

 enable us to understand how Professor Magnus obtained an 

 absorption of only 2^ per cent, with the blackened tube, and 

 as much as 14*75 per cent, with the unblackened one. With the 

 latter, the source and the plate of glass nearest the source send 

 a copious flux down the tube to the plate at the opposite end ; 

 the oblique rays are in great part reflected by the interior sur- 

 face, and thus reach the plate adjacent to the pile. With the 

 blackened tube this oblique radiation is entirely cut off, the rays 



