1833.] Attraction and Repulsion. 509 
Sir Isaac Newron, “ in two large, tall, cylindrical, vessels of glass 
inverted, two little thermometers be suspended, so as not to touch 
the vessels, and the air be drawn out of one of these vessels, and 
these vessels, thus prepared, be carried out of a cold place intoa 
warm one, the thermometer in vacuo will grow warm as much, and 
almost as soon, as the thermometer which is not in vacuo, And 
when the vessels are carried back into the cold -place, the thermo- 
meter in vacuo will grow cold, almost as soon as the other ther- 
mometer.”’ 
«« Ts not the heat of the warm room conveyed through the vacuum 
by the vibrations of a much subtiler medium than air, which, after 
the air was drawn out, remained in the vacuum? and is not this 
medium the same with that medium by which light is reflected and re- 
fracted? Anddo not hot bodies communicate their heat to contiguous 
cold ones, by the vibrations of this medium propagated from them into 
the cold ones? And is not this medium exceedingly more rare and 
subtle than air, and exceedingly more elastic and active ? And doth it 
not readily pervade all bodies* ?”’ 
If to these questions were added this one, ‘“‘ And is it not attracted 
by all particles of all bodies, but with various degrees of force in each ?”’ 
This medium would at once form the matter of calorific repulsion, 
and the phenomena of moving heat would arise from its motion and 
vibration, which must necessarily happen, both from its various affi- 
nities, and from its own elasticity tending to an equilibrium of force. 
Caloric, like this medium, exists, from the minuteness and mutual 
elasticity of its particles, in what isa vacuum to other bodies. By 
* It is a singular circumstance, that some late authors have quoted this pas- 
sage in order to shew, that Newton was doubtful about the nature of light, and 
seemed to accord with the theory of tremulous motions in an universal ether, 
rather than of moving particles emitted from bodies. It is certainly incredible 
that Sir I. Newron should at the end of his Treatise on Optics, introduce an opinion 
which would thus overthrow the whole doctrine he had been labouring to establish. 
Nor is it more probable that entertaining such an opinion, he should have written 
the 14th section of the 1st book of his Principia, which with it would be nothing more 
than vain and idle speculation. But the words of the query convey no such mean- 
ing. ‘They express an impression upon the author’s mind, that the phenomena of 
refraction, and reflection, are not the effect of attraction or repulsion exercised by 
the particles of the grosser bodies, commonly called mediums, upon the particles 
of light, but those of a very far more subtle medium interspersed between the par- 
ticles of the above-mentioned bodies. Nothing is said implying that this subtle 
fluid is light itself; on the contrary it is spoken of in a totally distinct character, 
as a medium, that is, as a substance having a boundary through which light finds 
a passage, or from the surface of which it is reflected. 
