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 all minerals, and many other bodies, takes its origin.
It is no part of my design to account to you, gentlemen, in what manner such an expansion of the parts of bodies can be brought about by the action of fire. Tho' perhaps a rotatory motion only of each particle on its own center might be sufficient to produce such a rarefaction; and the more so, if such parts were any other figures than spheres, as by the percussion of their angles they must result further from each other. Nor is the existence of such a rotatory motion without some probability, when we observe the verticillary motion given to charcoal-dust thrown on nitre in fusion, or the wonderful agitation of the parts of burning phosphorus, or even of a common red letter-wafer touched by the flame of a candle. But as in this paper I have laboured (and I hope not without success) to shew you, that some properties of solar heat are sufficient to account for the elevation and support of vapours; so in another letter I propose nearly to demonstrate to you, that the electric æther is far from having any share in the production of this important phænomenon.
From,
Gentlemen,
Your very humble Servant,
Erasmus Darwin.
Litchfield, Mar. 20, 1757.
LETTER II.
To the very honourable and learned the President and Members of the Royal Society.
Gentlemen,
Read May 5, 1757.
EVERY theoretical inquiry, whose basis does not rest upon experiments, is at once exploded in this well-thinking age; where truth, under your patronage, has at length broke thro' those clouds, with which superstition, policy, or parade, had overwhelmed her. But experiments themselves, gentlemen, are not exempted from fallacy. A strong inventive faculty, a fine mechanic hand, a clear unbiassed judgment, are at once required for the contrivance, conduct, and application, of experiments; and even where these are joined (such is the condition of humanity!) error too frequently intrudes herself, and spoils the work.