Radiation of Heat by Gases and Vapours. 193 



produced by benzole with a measure possessing only y^th of a 

 cubic inch in capacity ; and yet for equal tensions of 0*5 of an 

 inch, alcohol cuts off precisely twice as much heat as benzole. 

 There is also an enormous difference between alcohol and sul- 

 phuric ether when equal measures at the maximum density are 

 compared; but to bring the alcohol and ether vapours up to a 

 common tension, the density of the former must be many times 

 augmented. Hence it follows that when equal tensions of these 

 two substances are compared, the difference between them dimi- 

 nishes considerably. Similar observations apply to many of the 

 substances whose deportment is recorded in the foregoing Tables; 

 to the iodide and chloride of amyle, for example, and to the pro- 

 pionate of ethyle. Indeed it is not unlikely that with equal 

 tensions the vapour of a perfectly pure specimen of the sub- 

 stance last mentioned would be found to possess a higher 

 absorptive power than that of ether itself. 



It has been already stated that the tube made use of in these 

 experiments was of bras3 polished within, for the purpose of 

 bringing into clearer light the action of the feebler gases and 

 vapours. Once, however, I wished to try the effect of chlorine, 

 and with this view admitted a quantity of the gas into the ex- 

 perimental tube. The needle was deflected with prompt energy ; 

 but on pumping out *, it refused to return to zero. To cleanse 

 the tube, dry air was introduced into it ten times in succession ; 

 but the needle pointed persistently to the 40th degree from zero. 

 The cause of this was easily surmised : the chlorine had attacked 

 the metal and partially destroyed its reflecting power ; thus the 

 absorption by the sides of the tube itself cut off an amount of 

 heat competent to produce the deflection mentioned above. For 

 subsequent experiments the interior of the tube had to be re- 

 polished. 



Though no other vapour with which I had experimented pro- 

 duced a permanent effect of this kind, it was necessary to be 

 perfectly satisfied that this source of error had not vitiated the 

 experiments. To check the results, therefore, I had a length of 

 2 feet of similar brass tube coated carefully on the inside with 

 lampblack, and determined by means of it the absorptions of all 

 the vapours which I had previously examined, at a common ten- 

 sion of 03 of an inch. A general corroboration was all I sought, 

 and I am satisfied that the few discrepancies which the mea- 

 surements exhibit would disappear, or be accounted for, in a 

 more careful examination. 



In the following Table the results obtained with the blackened 

 and with the bright tubes are placed side by side, the tension 



* Dense dark fumes rose from the cylinders on this occasion ; a similar 

 effect was produced by sulphuretted hydrogen. 



