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 part of 
the full discharge, with a proportional diminution when the af- 
fected surlace 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 
OY he eS Mm — 
operation. 
It is easy, in any case, to ascertain the real direction of the 
aérial 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 from this approximating surface with an expansive sweep. 
‘On the contrary, when a body maintains either a higher or a 
jower temperature than that of the surrounding atmosphere, 
like the sentient ball of the pyroscope in the focus of a re- 
Hector fronting a charged canister, it receives and absorbs the 
impressions vibrated at a distance. ‘the 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- 
? ; . np ees vil 
quires free space or vacuity. But a body will either.lose 
gain 
lt rte os 
cag 
