92 
PEOFESSOE TYNDALL ON THE ABSOEPTION AND 
the screen S, and the compensating cube C' were used as in the other experiments. E 
is the end of the front chamber, and C the source of heat. In some experiments I 
had the end E closed by a plate of rock-salt, in others it was allowed to remain open; 
a distance of about 12 inches intervening between the radiating surface and the open 
end E' of the experimental tube. 
Closing the cocks Q and O, and opening Q' and O', gentle pressure being apphed to 
the bag B, a current of moist air was slowly discharged at the end E' of the experi- 
mental tube. The pump in connexion with A was then worked, and thus by degrees 
when the air was sucked into the tub§ T. The deflection of the galvanometer was 30*^, 
the moist moist air filled the tube as completely as the arrangement permitted*; this 
deflection being due to the predominance of the compensating cube over the radiating 
source C. 
The cocks Q' and O' were now closed, and Q and O opened ; proceeding as before, a 
current of dry air was discharged at E', and this air was drawn into the tube T in the 
manner just described. The moist air was thus displaced by dry; and, while the dis- 
placement was going on, the galvanometer was observed through the distant telescope. 
The needle soon commenced to sink, and slowly went down to zero ; proving that a 
greater quantity of heat passed through the dry than through the moist air. The wet 
air was substituted for the dry, and the dry for the wet twenty times in succession, with 
the same constant result ; the entrance of the humid air caused the needle to move from 
0° to 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 f. 
§13. 
Whence, then, arise those differences between Professor Magnus and myself 1 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; bnt a 
series of experiments executed with reference to this point so early as the 26th of July, 
1859, showed me that the accm’acy of the results was entirely compromised by bringing 
* Still, of course, only partially. 
t It is sheer want of time that prevents me from describing more particularly the numerous experiments 
executed with open tubes. 
