502 PHILOSOPHICAL TRANSACTIONS. [ANNO \7QQ. 



causing the ice in the bottle a to thaw, and suffering the 3 bottles to remain till 

 they and their contents had acquired the exact temperature of the surrounding air, 

 I wiped them very clean, and comparing them together, I found their weights 

 remained unaltered. This experiment I afterwards repeated several times, and al- 

 ways with precisely the same result; the water, in no instance, appearing to gain, 

 or to lose, the least weight, on being frozen, or on being thawed; neither were 

 the relative weights of the fluids in either of the other bottles in the least changed, 

 by the various degrees of heat, and of cold, to which they were exposed. If the 

 bottle were weighed at a time when their contents were not precisely of the same 

 temperature, they would frequently appear to have gained, or to have lost, some- 

 thing of their weights; — but this doubtless arose from the vertical currents which 

 they caused in the atmosphere, on being heated or cooled in it; or to unequal 

 quantities of moisture attached to the surfaces of the bottles; — or to both these 

 causes operating together. 



As I knew that the conducting power of mercury, with respect to heat, was 

 considerably greater than either that of water, or that of spirit of wine, while its 

 capacity for receiving heat is much less than that of either of them, I did not think 

 it necessary to inclose a thermometer in the bottle c, which contained the mercury; 

 for it was evident, that when the contents of the two other bottles should appear, 

 by their thermometers, to have arrived at the temperature of the medium in which 

 they were exposed, the contents of the bottle c could not fail to have acquired it 

 also, and even to have arrived at it before them; for the time taken up in the 

 heating or in the cooling of any body is, caeteris paribus, as the capacity of the 

 body to receive and retain heat directly, and as its conducting power inversely. 

 The bottles were suspended to the balance by silver wires, about 1 inches long, 

 with hooks at the ends of them ; and, in removing and changing the bottles, I 

 took care not to touch the glass. I likewise avoided on all occasions, and particu- 

 larly in the cold room, coming near the balance with my breath, or touching it, or 

 any part of the apparatus, with my naked hands. 



Having determined that water does not acquire or lose any weight, on being 

 changed from a state of fluidity to that of ice, and vice versa, I shall now take my 

 final leave of a subject which has long occupied me, and which has cost me much 

 pains and trouble; being fully convinced, from the results of the above-mentioned 

 experiments, that if heat be in fact a substance, or matter, — a fluid sui generis, as 

 has been supposed, — which, passing from one body to another, and being accumu- 

 lated, is the immediate cause of the phenomena we observe in heated bodies, (of 

 which, however, I cannot help entertaining doubts), it must be something so infi- 

 nitely rare, even in its most condensed state, as to baffle all our attempts to discover 

 its gravity. And if the opinion which has been adopted by many of our ablest 

 philosophers, that heat is nothing more than an intestine vibratory motion of the 

 constituent parts of heated bodies, should be well founded, it is clear that the 

 weights of bodies can in no wise be affected by such motion. It is, no doubt, on 



