48 Prof. Tyndall on the Passage of Radiant Heat 



"At the conclusion of the experiments I found that the de- 

 flection due to the total radiation was 86°'2. 



"I also saw the following experiments made by yourself when 

 the ends of the brass tube were closed by plates of rock-salt : — 



Permanent deflection 

 of needle. 



o 



Tube filled with dry air ... . +7 

 After exhaustion of tube .... — 3 

 After admission of laboratory air . — 42 



" Rain was falling at the time these last determinations were 

 made, and the air was very moist. On removing the plates of 

 rock-salt from the tube they appeared to be quite dry ; and after 

 being breathed upon, the film of moisture soon disappeared and 

 they recovered their previous lustre. I ought perhaps to mention 

 that these experiments are not selected, they are the only ones I 

 have made upon the subject, and they were performed in the 

 sequence given above : after a very careful scrutiny I have been 

 unable to detect any source of fallacy in them, and they there- 

 fore appear to me to prove conclusively that obscure radiant heat 

 passes much more readily through dry than through moist air. 



(t In conclusion, I cannot but express my surprise and admi- 

 ration at the precision and sharpness of the indications of your 

 apparatus. Without having actually worked with it I should 

 not have thought it possible to obtain these qualities in so high 

 a degree in determinations of such extreme delicacy, and which 

 are so well known to be exposed to numerous sources of de- 

 rangement. 



" Believe me, 

 " Royal Institution, " Yours very truly, 



June 19, 1863." " E. FRANKLAND." 



" P.S. Since writing the foregoing letter, I have repeated the 

 experiments there recorded without any source of heat at either 

 end of the pile, in order to ascertain whether the introduction 

 and withdrawal of dry air at all affected the galvanometer. The 

 tube was first full of the common air of the laboratory, and the 

 needle remained steadily at + 12*5 for a quarter of an hour. A 

 current of moistened air was now drawn through the tube for ten 

 minutes in precisely the same manner as when the two sources 

 of heat were employed, the needle being closely watched during 

 the whole time. It oscillated between + 12 and +13, but never 

 passed these limits on either side. The current was now inter- 

 rupted and the needle closely watched for five minutes : it re- 

 mained perfectly steady at 12*5. A current of dried air was 

 now conveyed through the tube for ten minutes; the needle 

 oscillated as before between 12° and 13°. 



