474- 



ON IMPRESSIONS OF COLD 



to half of the whole discharge, and from a surface of polished 

 silver, it exceeds not the twentieth part of the entire expendi- 

 ture. But such are the accelerating effects of a current of air, 

 that, in a high wind, the pulsations of heat darted from a vitre- 

 ous and a metallic surface may not form the twentieth and the 

 two hundredth part. In a medium of hydrogen gas, those 

 pulsations are comparatively feebler, reaching, in the case of a 

 stagnant atmosphere, only to the eighth or the eightieth rart of 

 the full discharge, with a proportional diminution when the af- 

 fected surface is exposed to the action of a current. 



The influence of the pulsatory emission of heat or cold, has, 

 therefore been greatly exaggerated. It comes merely as an auxi- 

 liary to the other modes of restoring the equilibrium of the ig- 

 neous fluid, and it often contributes a very small share only to- 

 wards the general effect. But whenever a body, left to itself 

 in the atmosphere, is observed to change its temperature, 

 some pulsatory action may be presumed to combine with the 

 operation. 



It is easy, in any case, to ascertain the real direction of the 

 aerial pulses. Whether a substance acquires or loses heat, it al- 

 ways approaches to the temperature of the ambient medium 

 by the very same process. The hot or cold pulses are pro- 

 jected/row this approximating surface with an expansive sweep. 

 On the contrary, when a body maintains either a higher or a 

 lower temperature than that of the surrounding atmosphere, 

 like the sentient ball of the pyroscope in the focus of a re- 

 flector fronting a charged canister, it receives and absorbs the 

 impressions vibrated at a distance. f i he intervention of a pa- 

 per screen does not prevent the spontaneous emission of hot 

 or cold pulses, but will effectually obstruct their passage from 



a remote object. 



Some theorists have imagined, that the escape of heat re- 

 quires free space or vacuity. But a body will either lose or 



gain 



