[ *45 ] 
opake body in nature. And from this we may col- 
led:, that tranfparent bodies are only heated at their 
furfaces, and that perhaps in proportion to their 
quantity of refradion : which will further give and 
receive illufiration from thofe very curious experi- 
ments, of producing cold by the evaporation of li- 
quors, publifhed by the learned Dr. Cullen, in the 
late volume of Effays Phyfical and Literary, at Edin- 
burgh. In thefe experiments a fpirit-thermometer 
was immerfed in fpirit of wine, and being fuddenly 
retraded, was again expofed to the air ; and as the 
fpirit of wine adhering to the glafs evaporated, the 
fpirit contained within the thermometer was obferved 
to fubfide. Now as the difference of the refradion of 
fpirit of wine and glafs is exceedingly minute, com- 
pared with the difference of refradion of fpirit of 
wine and air we may confider, in the above experi- 
ment, the heat to be communicated to the thermo- 
meter only at its furface : but here the adherent fluid 
efcapes as foon as heated ; by which means the glafs, 
and its contents, are deprived of that conftant addi- 
tion of heat, which other bodies perpetually enjoy 
either from the fun-beams immediately, or from the 
emanations of other contiguous warmer bodies ; and 
muff: thence, in a few minutes, became colder than 
before. 
The ingenious Mr. Eeles, I dare fay, has already 
forefeen the ufe lam going to make of this principle y 
viz. “ That the little fpherules of vapour will thus, 
<f by refrading the folar rays, acquire a conftant 
“ heat, tho’ the furrounding atmofphere remain 
4< cold.” And as from the minutenefs of their dia- 
meters, if they are allowed to be globules, they mufl 
