430 Prof. Tyndall on the Absorption and 



30°, while the entrance of dry air caused it to fall from 30° to 

 0°. The air-pump was resorted to, because I found that when I 

 attempted to displace the air by the direct force of the current 

 from B, the temperature of the pile, or of the source, was so 

 affected by the fresh air as to confuse the result. I may remark 

 that not only have I operated thus for days with aqueous vapour, 

 but every result which I have obtained with vapours generally has 

 been thus confirmed, so that all doubt as to the applicability of 

 the rock-salt plates to researches of this nature may, I think, be 

 abandoned*. 



§ 13. Whence, then, arise those differences between Professor 

 Magnus and myself? I have no doubt that every one of his 

 published results is the record of an experiment made with the 

 utmost care which it is possible to bestow upon scientific work. 

 The differences between us are, I imagine, to be referred to a 

 radical defect in his apparatus. His desire was to do away with 

 plates of all kinds between his source of heat and his pile, and 

 hence he brought his gas into direct contact with his source of heat. 

 The same thought had occurred to myself, and I was on the 

 point of falling into the same error ; but a series of experiments 

 executed with reference to this point, so early as the 26th of July 

 1859, showed me that the accuracy of the results was entirely 

 compromised by bringing the gas to be examined into contact 

 with the source. I obtained thus an action forty times what I 

 knew it ought to be, and was confirmed in the view which caused 

 me to interpose a vacuous chamber in front of the experimental 

 tube. Let me here record a few experiments made on the 4th of 

 last November in connexion with this subject. 



I first satisfied myself that the drying-apparatus was in perfect 

 condition, the air of the laboratory producing, when sent through 

 it, an absorption of 1. This same air was sent into the front 

 chamber, that is, into direct contact with the source. The galva- 

 nometer needle moved as it does in the case of absorbent gases, 

 and at the end of two minutes declared a loss of heat equivalent 

 to an absorption of 50. The front chamber is 8 inches in length ; 

 the experimental tube is 33 inches long ; hence a column of 

 8 inches, in contact with the radiating surface, produced at least 

 fifty times the effect of a column more than four times as long 

 when the air was separated from the radiating surface. 



I made the foregoing experiment three times in succession, 

 and after two minutes found the needle pointing to precisely the 

 same degree ; the lowering of the source was perfectly constant 

 and regular, and in all cases showed a loss equivalent to an 

 absorption of 50. 



* It is sheer want of time that prevents me from describing more par- 

 ticularly the numerous experiments executed with open tubes. 



