VOL. L.] PHILOSOPHICAL TRANSACTIONS. 127 



the superincumbent fluid, greatly less than thatof boiling water ; would instantly 

 disperse the whole so luuited into vapour. Secondly, that in the opinion of Sir 

 Isaac Newton, well illustrated by the late lamented Mr. Melvil, the sun-beams 

 appear only to communicate heat to bodies by which they are refracted, reflected, 

 or obstructed ; whence by their impulse, a reaction or vibration is caused in the 

 parts of such impacted bodies. 



This is supported by the experiment of approaching some light body, or blow- 

 ing smoke near the focus of the largest glasses ; and from observing that these 

 do not ascend, it is evident that the air is not so much as warmed by the passage- 

 of those beams through it, yet they would instantly calcine or vitrify every opaque 

 body in nature. And from this we may collect, that transparent bodies are only 

 heated at their surfaces, and that perhaps in proportion to their quantity of re- 

 fraction ; which will further give and receive illustration from those very curious 

 experiments, of protlucing cold by the evaporation of liquors, published by Dr, 

 Cullen, in the late volume of Essays Pliysical and Literary, at Edinburgh. In 

 these experiments a spirit-thermometer was immersed in spirit of wine, and being 

 suddenly retracted, was again exposed to the air ; and as the spirit of wine ad- 

 hering to the glass evaporated, the spirit contained within the thermometer was 

 observed to subside. Now as the difference of the refraction of spirit of wine 

 and glass is exceedingly minute, compared with the difference of refraction of 

 spirit of wine and air ; we may consider in the above experiment, the heat to be 

 communicated to the thermometer only at its surface : but here the adherent 

 fluid escapes as soon as heated ; by which means the glass, and its contents, are 

 deprived of that constant addition of heat, which other bodies perpetually enjoy 

 either from the sun-beams immediately, or from the emanations of other conti- 

 guous warmer bodies ; and must thence, in a few minutes, become colder than 

 before. 



The use to be made of this principle, is this : * That the little spherules of va- 



* pour will thus, by refracting the solar rays, acquire a constant heat, though 



* the surrounding atmosphere remain cold.' And as for the minuteness of their 

 diameters, if they are allowed to be globules, they must do this to a very great 

 degree, he apprehends none of these objections will take place, with which Mr. 

 Eeles has so sensibly confuted the former received theories on this subject. If 

 it be asked, how clouds are supported in the absence of the sun ? it must be re- 

 membered, that large masses of vapour must for a considerable time retain 

 much of the heat they have acquired in the day; at the same time reflecting how 

 small a quantity of heat was necessary to raise them ; and that doubtless even a 

 less will be sufficient to supj^ort them, as from the diminished pressure of the 

 atmosphere at a given height, a less power may be able to continue them in 



