in Fluids. 371 



touch it; but It can never give us any information re- 

 specting the relative temperatures of those particles of air. 



If, during the most intense frost, a thermometer were 

 suspended in the neighbourhood of a burning candle, — 

 in the same room, for instance, — if it were placed over 

 the candle, or nearly so, though it should be distant from 

 it several feet, as air is a non-conductor of Heat, there 

 is not the smallest doubt but that some solitary particles 

 of air, heated by the candle to the intense Heat of melt- 

 ing gold, would reach the thermometer; but neither the 

 thermometer, nor the hand held in the same place, could 

 give any indication of such an event. 



As it appears, from all that has been said, that intense 

 heat may exist even under the form oi sensible Heat, where 

 its presence cannot be discovered or detected by us ; and 

 as it seems highly probable that in many cases, where its 

 existence may escape our observation, it may neverthe- 

 less be capable of producing very visible effects, I think 

 we ought always to be much on our guard in accounting 

 for effects similar to those which are known to be pro- 

 duced by Heat, and never, without very sufficient rea- 

 sons, attribute them to the agency of any other unknown 

 power ; and this caution appears to me to be peculiarly 

 necessary in accounting for those effects which have been 

 found to be produced in various bodies when they are 

 exposed to the action of the sun's rays. 



If the solar rays concentrated in the focus of a lens, 

 when they are made to fall on a piece of wood, instantly 

 change its surface to a black colour, and reduce it to 

 charcoal, why may we not conclude that the change of 

 colour which is gradually or more slowly produced in the 

 same kind of wood when it is simply exposed in the 

 sunbeams is produced in the same manner ? 



