, 
Second Letter from Dr. Hare to Prof. Faraday. an $ 
_ the consequent density of the calorific atmospheres existing in 
the mass which undergoes these changes. 
Such being the constitution of expansible bodies, agreeably 
to the hypothesis in question, it seems to me that the process, 
by which caloric is propagated through them by conduction, must 
be extremely different from that by which it is transmitted from 
one part of space to another by radiation. In the one case the 
calorific particle flies, like a cannon ball, with an inconceivably 
greater velocity, which is not sensibly fetarded by the reflecting | 
or refracting influence of intervening transparent media: in the — 
other case it must be slowly imparted from one calorific atmos- & 
phere to another, until the repulsion sustained on all sides isin — 
equilibrio. It is in this way that I have always explained the s 
fact that metals are bad radiators, while good reflectors.+ * 
*T subjoin the language which I er held respecting the constitution of expan- 
sible solids, during the last twent 
“The expansion of matter, whether solid, ‘nt or aériform, by an increase of 
pune may be thus explain ned. 
“In proportion as the temperature - within ; ey space is raised, there will be more 
calorie i in the pred of the particles of any mass contained in the space. The 
more caloric may abound in the vicinity of the particles, the more of it will com- 
bine with them ; sti in to the quantity of caloric thus combined, will 
they be actuated by that reciprocally Foon power, which, i In proportion toits 
intensity, re their distance from each other. e 
“ There may be some analogy between the mode i which each ponderable atom 
is wsiiietiadild by the caloric which it attracts, and that in which the earth is 
rounded by the atmosphere ; and as in the latter case, so probably in the 
the density is inversely as the square of the distance. 
‘“ At a height at which the atmospheric pressure does not exceed a grain to the 
supply of air from some remote region ; is it not evident that a condensation would 
ensue in all the inferior strata of the atmosphere, until the pressure would be 
doubled throughout, so as to become at the terrestrial surface, 30 Ibs. instead of the 
present pressure of 15 Ibs.? Yet the pressure at the point from which the change 
would be propagated, would not exceed two grains per square inch. In like man- 
ner, it may be presumed that the atmospheres of caloric are increased in quan tity 
and density about their bec nati atoms, by a slight increase in the calorific ten- 
sion of the external me 
t I will here quote the pactahe which has been given in my lectures for the last 
twenty years. ‘“ Metals appear to consist of particles so united with each other, or 
with caloric, as to leave no pores through which radiant caloric can be projected. 
Hence the only portion of any metallic mass which can yield up its rays by radia- 
tion, is the external stratum. 
< On the other hand, from its porosity, and pro 
oric within its pores tenaciatisly as an ingredient in its composition, ~~ oppet * 
ses but little obstruction to the passage of that subtile principle, when in the radi- 
ant form ; and hence its particles may all be simultaneously engaged in radiating 
ie 
ae 
