On the Cause of Heat. 105 
such as losing its motion when combining with bodies, producing 
motion when transmitted from one body to another, aud gaining 
projectile motion when passing into free space; so that many 
hypotheses must be adopted to account for its agency, which 
renders this view of the subject less simple than the other. Very 
delicate experiments have been made, which show that bodies, 
when heated, do not increase in weight. This, as far as it goes, 
is an evidence against a subtile elastic fluid, producing the calo- 
rific expansion ; but it cannot be considered as decisive on ac- 
count of the imperfection of our instruments. A cubical inch of 
inflammable air requires a good balance to ascertain that it has 
any sensible weight, and a substance bearing the same relation 
to this, that this bears to platinum, could not perhaps be weighed 
by any method in our possession.” 
These suggestions of Sir H. Davy’s are to me unsatisfactory. 
It is fully established in mechanics, that when a body in mo- 
tion is blended with and thus made to communicate motion to 
another body, previously at rest, or moving slower, the velocity 
of the compound mass after the impact will be found, by multi- 
plying the weight of each body by its respective velocity, and 
dividing the sum of the products by the aggregate weight of 
both bodies. Of course it will be more than a mean or less than 
a mean, accordingly as the quicker body was lighter or heavier 
than the other. Now, according to Sir Humphry Davy, the par- 
ticles of substances which are unequally heated are moving with 
unequal degrees of velocity: of course when they are reduced by 
contact to a common temperature, the heat, or, what is the same 
(in his view), the velocity of the movements of their particles, 
ought to be found by multiplying the heat of each by its weight 
and dividing the sum of the product by the aggregate weight. 
Hence if equal weights of matter be mixed, the temperature ought 
to be a mean; and if equal bulks, it ought to be as much nearer 
the previous temperature of the heavier substance as the weight 
of the latter is greater ; but the opposite is in most instances 
true. When equiponderant quantities of mercury and water are 
mixed at different temperatures, the result is such as might be 
expected from the mixture of the water, were it twenty-six times 
heavier ; so much nearer to the previous heat of the water is the 
consequent temperature, It may be said that this motion is not 
measurable upon mechanical principles, How then, 1 ask, does 
it produce mechanical effects?) These must be produced by the 
force of the vibrations, which are by the hypothesis mechanical : 
for whatever laws hold good in relation to moving matter in 
mass, must operate in regard to each particle of that matter ; the 
effect of the former can only be a multiple of that of the latter. 
Indeed, one of Sir Humphry Davy’s reasons for thinking heat to 
Vol, 59. No, 286, Feb, 1822, O consist 
