312 Lord Rayleigh on the Cooling of Air by Radiation 



At this rate the time of cooling of a mass of air must exceed, 

 and probably largely exceed, 60 seconds. To suppose that 

 this time is one-twelfth of a second would require a sphere of 

 air 2 millim. in diameter to radiate as much heat as if it were 

 of blackened copper at the same temperature. 



Although, if the above argument is correct, there seems 

 little likelihood of the cooling of moderate masses of air being 

 sensibly influenced by radiation, I thought it would be of 

 interest to inquire whether the observed cooling (or heating) 

 in an experiment on the lines of Clement and Desormes 

 could be adequately explained by the conduction of heat from 

 the walls of the vessel in accordance with the known con- 

 ductivity of air. A nearly spherical vessel of glass of about 

 35 centim. diameter, well encased, was fitted, air-tight, with two 

 tubes. One of these led to a manometer charged with water or 

 sulphuric acid ; the other was provided with a stopcock and 

 connected with an air-pump. In making an experiment the 

 stopcock was closed and a vacuum established in a limited 

 volume upon the further side. A rapid opening and re- 

 closing of the cock allowed a certain quantity of air to 

 escape suddenly, and thus gave rise to a nearly uniform 

 cooling of that remaining behind in the vessel. At the same 

 moment the liquid rose in the manometer, and the obser- 

 vation consisted in noting the times (given by a metronome 

 beating seconds) at which the liquid in its descent passed 

 the divisions of a scale, as the air recovered the temperature 

 of the containing vessel. The first record would usually be 

 at the third or fourth second from the turning of the cock, 

 and the last after perhaps 120 seconds. In this way data 

 are obtained for a plot of the curve of pressure; and the 

 part actually observed has to be supplemented by extra- 

 polation, so as to go back to the zero of time (the moment of 

 turning the tap) and to allow for the drop which might occur 

 subsequent to the last observation. An estimate, which 

 cannot be much in error, is thus obtained of the w r hole rise in 

 pressure during the recovery of temperature, and for the 

 time, reckoned from the commencement, at which the rise 

 is equal to one-half of the total. 



In some of the earlier experiments the w r hole rise of 

 pressure (fall in the manometer) during the recovery of 

 temperature was about 20 millim. of water, and the time of 

 half recovery was 15 seconds. I was desirous of working 

 with the minimum range, since only in this way could it be 

 hoped to eliminate the effect of gravity, whereby the interior 

 and still cool parts of the included air would be made to 

 fall and so come into closer proximity to the walls, and thus 



