414 Cold in the higher [Book V. 



that the a<5tion of light is capable of producing heat in 

 bodies ; and the equally well eftablimed fact, that the 

 action of light upon a tranfparent medium, through 

 which it is eafily tranfmitted, is extremely feeble. 

 This may be proved by the eafieft of all poffible ex- 

 periments. If highly rectified fpirit is inclofed in a 

 pure glafs phial, or any perfectly tranfparent veffel, the 

 rays of light concentrated by the moft powerful burn- 

 ing glafs, will not inflame it ; if, however, the fpirit 

 is placed in a fpoon, or if that part of the tranfpa- 

 rent veffel which is not oppofed to the burning glafs, 

 is coated with paint or any fubftance, which intercepts 

 die rays of light, imbibes or condenfes them, the fpi- 

 rit will be inftandy fet in a blaze. The earth is there- 

 fore the great receptacle of heat, where it is abfdrbed 

 and kept as in aftore-houfe; but the furface of earth 

 which is expofed to the fun on the tops of mountains 

 is but very fmall, and cannot imbibe much of the fun's 

 heat. The rays from the fun can indeed only ftrike 

 the different fides of the mountain for a fhort period 

 in every day, and in fome days and in fome parts, not 

 at all. " A horizontal plain alfo when the day is clear, 

 is expofed at mid-day to the perpendicular and undi- 

 minimed action of the fun's rays, while they fall ob- 

 liquely on a plain which is much inclined, or on a pile 

 of rocks." In theie elevated fituations, therefore, the 

 majority of the fun's rays pafs through a tranfparent 

 medium, as through the fpirit inclofed in a clear glafs 

 veflel j and there is a mafs of opake matter to collect or 

 condenfe the heat. The atmofphere, therefore, in 

 thofe regions is neceflarily colder than in thofe which 

 approach nearer the furface of the earth. 



The cold in thefe higher regions of the atmofphere, 

 may be one caufe why vapours are "not collected fo 

 plentifully there as nearer the earth. The clouds are 



feldom 



