284 Prof. Tyndall on the Absorption and 



pared 27 per cent. ; while a layer so opake that it cut off the 

 beams of the sun itself, transmitted 23 per cent, of the rays 

 emitted by a source heated to 100° C. 



At page 93 of La Thermochrose, Melloni examines the absorp- 

 tion of this substance for all sorts of rays, and by a series of 

 ingenious experiments, and reasonings remarkable for their 

 clearness and precision, he arrives at the conclusion that lamp- 

 black absorbs with the same intensity all descriptions of radiant 

 heat*. At page 284, however, he cites and discusses with the 

 same precision a series of experiments made with smoked rock- 

 salt, in which he shows that the same layer of lampblack trans- 

 mits 8 per cent, of the rays from a lamp of Locatelli, 10 per 

 cent, of those of incandescent platinum, 18 per cent, of those 

 from copper heated to 400° C, and fully 23 per cent, of those 

 emitted by a source of 100° C. Now a transmission of 8 per 

 cent, implies an absorption of 92 ; while transmissions of 10, 18, 

 and 23 per cent, imply absorptions of 90, 82, and 77. But 

 that the self-same layer of lampblack absorbs 77 per cent, of 

 the rays from one source and 92 per cent, of the rays from 

 another, is at variance with the statement that lampblack absorbs 

 heat from all sources with the same intensity. Suppose the 

 surface of a thermo-electric pile to be coated by a layer of lamp- 

 black of the same thickness as that which coated Melloni' s plate 

 of salt ; 23 per cent, of the rays from a source of 100° C. would 

 go right through such a layer and impinge upon the metal face 

 of the pile ; and if the latter were a good reflector, the heat inci- 

 dent upon it would be in great part retransmitted through the 

 lampblack and lost to the instrument. For a source of 100° C._, 

 this loss would be many times greater than for a Locatelli lamp. 

 Possibly, however, Melloni meant simply to assert that for prac- 

 tical purposes the absorption by the face of his pile might be 

 considered to be the same for all kinds of heatf. 



§5.1 have now to record some new experiments on the action 

 of vapours upon radiant heat. A number of glass flasks were 

 prepared, of the shape and size of common test-tubes, each of 

 which was furnished with a brass cap carefully cemented on to 

 it. By means of this it could be attached to a stopcock, and 

 thus connected with the experimental tube. The mode of opera- 

 tion was this. The liquid was introduced into the flask by means 



* " Done, le noir de fumee absorbe avec la meme intensite toute sorte 

 de rayonnements calorifiques" (p. 101). 



t The sun, through the floating carbon of the London atmosphere, some- 

 times presents a most instructive appearance. Entirely shorn of his rays 

 and of perfectly uniform brightness, his colour at times is as red as blood. 

 This is doubtless mainly due to the comparative transparency of the float- 

 ing carbon for the longer undulations. 



