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thought very much resembled the motions of insects; though, when he endea¬ 
voured to catch them, he only found luminous spots upon his handkerchief, 
which were enlarged when he pressed them with his linger. 
The dissimilarity of light and heat is evinced by this simple circum¬ 
stance; that as light gives no heat to transparent bodies, which the emana¬ 
tions from a fire do, there is reason to believe them to be different fluids. 
Thus when smoke is blown near the focus of a large burning glass, it does 
not ascend; which shews that the air is not heated and rarefied by it; though 
it would burn or vitrify in an instant any opake body which might be opposed 
to it; but the emanations of heat from a fire soon rarefy and warm the air in 
its vicinity, causing it to ascend, as may be seen by a spiral card-vann placed 
over a chimney-piece, and which is agreeably seen in the use of the new glass 
fire-screens of Parisian invention, which placed before a parlour fire permit 
the rays of light to pass, but intercept the emanations of fluid heat. 
For the better imbibing of caloric, or the matter of heat, the vegetable 
mould is of a dark colour nearly black, and that this is most favourable for 
that intention is shewn by the following experiment. 
EXPERIMENT. 
During the hot weather, which we had in the latter end of June and the 
beginning of July, 17 ^ 2 , I made an experiment at Cambridge, which I then 
thought no more of, but which an accident brought to my mind again; and 
I now venture to relate an account of it, in hopes that some philosophical 
friend will take the trouble of prosecuting it. I exposed, says the Bishop of 
LandafF, the bulb of an excellent thermometer to the direct rays of the sun, 
when the sky was perfectly free from clouds; the mercury rose 108° of Fah¬ 
renheit’s scale, and continued stationary. A fancy struck me, to give the 
bulb a black covering; this was easily effected by a cameFs hair pencil and 
Indian ink; the mercury fell a few degrees during the application of the coat¬ 
ing, and the evaporation of the water; but presently after rose to 118°, or 
10° in consequence of the black coat with which I had covered that part of 
the bulb which was exposed to the sun. 
The air, which is a bad conductor of electricity, is known also to be a bad 
conductor of heat; and thence prevents the heat acquired from the sun’s rays 
3 C 
