The ingenious Mr. Eeles, I dare say, has already foreseen the use I am going to make of this principle; _viz._ “That the little spherules of vapour will thus, by refracting the solar rays, acquire a constant heat, tho’ the surrounding atmosphere remain cold.” And as from the minuteness of their diameters, if they are allowed to be globules, they must do this to a very great degree, I apprehend none of those objections will take place against us, with which Mr. Eeles has so sensibly confuted the former received theories on this subject. If we are asked, how clouds come to be supported in the absence of the sun? It must be remembered, 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 support them, as from the diminished pressure of the atmosphere at a given height, a less power may be able to continue them in their present state of rarefaction; and, lastly, that clouds of particular shapes will be sustained or elevated by the motion they acquire from winds. I should here have concluded this paper, perhaps already too long; but upon revising it, I find, where the affinity of some bodies with heat is mentioned, that the deductions made from thence are not sufficiently explained to be intelligible. First then, If the power of expansion of any two bodies, by heat, be in a greater proportion than their specific gravities, then will there be a certain degree of heat, in which their specific gravities will be equal; and another, in which the gravity of that, which was lighter when cold, will exceed the gravity of that, which was heavier when cold. Hence zinc and bismuth alter their specific gravities in fusion; some urine, and many solutions of solids, grow turbid as they cool; others alter their colours. Secondly, If (the power of expansion by heat being equal) the power of retaining heat be in a greater ratio than the specific gravities; then, during the time of cooling after being sufficiently heated, there will be an instant, when the heavier body will become the lighter, and swim upon the other. This seems the case in the buff covering of inflamed blood, the skum of heated milk, and the cristallization of some salts: for if these effects were from the evaporation of the thinner parts at the surface, they should happen during the greatest evaporation, or when boiling; but, on the contrary, they are all done in the greatest degree when the liquor has for some time began to cool. Lastly, If the quickness of acquiring heat be in a greater proportion than their specific gravities (the power of expansion being equal), then, during the time of their acquiring heat, there will be an instant, when the body, that was heavier when cold, will now become the lighter. From one or more of which principles, I apprehend, the volatility or fixity of