362 Prof. Olinsted’s Reply to Dr. Hare: 
not mean to assert such a doctrine, then why does he con- 
ceive it necessary to suppose a fluid upon which the phenom- 
ena of repulsion depend,—* in which the self repellent pow- 
er resides’”’—distinct from the bodies themselves, which ex- 
hibit such repulsion? Moreover, if caloric be identical 
with the Fe ng of repulsion, or be the repellent principle 
itself, how will Dr. H. explain the fact that caloric sometimes 
increases the attraction of bodies foreach other? In respect 
to solids, he might argue that repulsion operates in first over- 
coming the cohesion of the particles for each other, and then 
leaving them at liberty to enter into combination ;* but what 
would he say of the fact that the attraction ef two gases, when 
there is no cohesion, is sometimes increased by heat? I am 
still inclined to think, that it will be found very difficult to 
prove, that the phenomena of repulsion depend on the me- 
chanical agencies of a fluid, or that that fluid is caloric. 
“In support of my opinion, (continues the Doctor) I also cited 
the radiation of heat in vacuo, agreeably to an experiment ef Sir 
ror, is influenced by a hot body in the focus of a mirror, the 
whole being within an exhausted receiver. I will thank Profes- 
sor Olmsted to expiain how heat can be transmitted aan such 
circumstances, even = eee ease than in pleno, if the cause of 
it be not material.” p. 5 
All that can be Lue from the radiation of heat in vac- 
uo is, that the radiation is not dependent on the presence of 
air. my overthrow the doctrine of Mr.-Leslie that, in 
radiation, heat is transmitted by aerial pulsations ; but I can- 
not see how the fact that heat is not dependent on air for its 
communications, proves that it is a material substance ; nor 
does there appear to be any more difficulty in conceiving why 
a heated body should communicate its influence to another 
body without the aid of air, than why the sun should com- 
TMunicate his attractive influence to Saturn or Uranus, with- 
ont the aid of such a medium. I cannot tell why a heated 
body should act through a vacuum, nor a Dr. Hare telt 
why it should act through air. We must, I think, confess 
ur ignorance of the modus operandi, both of attraction and 
_Fepulsion. ‘The Doctor proceeds : 
lo ach cas, does the repulsion cease at the moment when the attrac- 
is it 
