484: Prof. Tyndall on the Action of Free Molecules on 



tube without coming iuto contact either with the surface of 

 the tube itself, or with any coating or lining of that surface. 

 With this apparatus all my old experiments on vapours have 

 been frequently repeated. There is no substantial difference 

 between the results thus obtained and those obtained with an 

 experimental tube where nineteen twentieths of the heat 

 which reached the pile was reflected heat." 



The tube referred to in this extract was of rough brass, 

 tarnished within. Hence, when air entered it, after exhaus- 

 tion, the dynamic heating of the tube and the partial conden- 

 sation of the vapour when the air was moist, produced an 

 amount of radiation from its internal surface which, though 

 small, was a source of some disturbance. In my present 

 experiments, therefore, another tube, of the dimensions above 

 given, is employed; and to reduce to a minimum any radiation 

 from its internal surface, it is coated within with silver de- 

 posited electrolytically, and highly polished. Experiments 

 with this tube show that it does not in any way disturb the 

 true radiation from the source. 



In Table I. are recorded five series of measurements exe- 

 cuted with a brass tube polished within. But as the liquids, 

 though reported pure by the manufacturing chemist, van- 

 slightly from time to time, I thought it advisable to adhere to 

 the same samples in experiments wherein a tube with reflecting 

 interior is to be compared with one permitting of no reflec- 

 tion. The following five series of new measurements were 

 therefore executed with the tube first mentioned. 



Table III. 

 Brass tube with internal reflection. 



A B C D E 



PrPsmivefi S P iral S P iral Lime > Lime > Lime ' 



±reseuie„. dark brigllt free> m i rr0 ]-. lens. 



Bisulphide of carbon . -48 O'O 31 3-4 3"1 2"5 



Chloroform .... -36 79 5"3 5-6 4-8 5"0 



Benzol -32 9-6 86 79 69 6-3 



Iodide of methyl . . . -46 12-1 8"8 10-2 8-1 75 



Iodide of ethvl ... -36 150 11-9 125 113 10-6 



Amylene -26 219 15-6 160 144 12"5 



Sulphuric ether ... -28 309 231 227 19"8 181 



Acetic ether ... -29 369 27 '3 29-6 24-1 



Formic ether .... '36 296 25-0 23-8 



Column A in this table contains the absorptions of the 

 respective vapours, in hundredths of the total radiation, when 

 the source was a platinum spiral just under incandescence, un- 

 aided by either lens or mirror. Under B are the absorptions 

 when the source was the same spiral heated to bright rednes-. 



