218 W. A. Norton on Molecular Physics. 
less, by the molecular atmospheres which they encounter. It 
may be urged as an objection to the notion that radiant heat is 
a repulsive force, that bodies when heated do not exert any 
sensible repulsive action upon other contiguous bodies. Weare 
not prepared to admit that experiment has furnished no evidence 
of such repulsive action, under any circumstances, but the entire 
ce of such action upon bodies of sensible magnitude would 
in fact be no decisive proof that waves of radiant heat do not 
convey a series of preponderating repulsive impulses; for if 
these impulses penetrate to the atoms of the molecules, they 
should be ultimately taken up by their atmospheres, and ex- 
nded as an expansive force upon these atmospheres, and in urg- 
ing the molecules asunder; and if they do not reach the atoms, 
no motion should be imparted to them. Since it is improbable 
that the more intense impulses of heat will be wholly absor 
by the particles immediately at the surface of the body upon 
which they fall, a direct repulsive action of the heat may take 
ect to a certain depth below the surface. Have we not in the 
a state of liquids evidence of such action exerted by 
the radiant heat from the hot vessel upon the liquid resting upon 
its interior surface? ; 
It is only when the heat-waves impinge upon isolated parti- 
es, or a small group of particles, that a progressive motion 
should be imparted. This supposition is apparently realized in 
the case of cometary matter repelled by the sun.' 
Light. 
The question of the — relation which the two physical 
agents, light and heat, bear to each other, has not been defini- 
tively settled; but the weight of evidence preponderates very 
decidedly in favor of the doctrine of their essential identity. 
The only “formidable outstanding objection” to this view con- 
sists in the fact that a strong light may be obtained which has 
little, if any, heating power.’ According to Melloni, the greet 
ish light, obtained by transmission of white light through 4 
uliar species of green glass colored by oxyd of copper, “ex 
its no calorific action capable of being rendered perceptible 
tra- 
ted by lenses as to rival the direct rays of the sun in brilliancy:” 
* In the article by the author, “On the Theoretical Determination of the Di- 
ons of Donati’s Comet,” (see this Journal, vol. xxxii, No. 94, p. 54, de.) it i8 
4 rigorous calculation, that the particles o disseminated over 
the breadth o the tail of the comet, were exposed to a force of re 
sun, of various degrees of intensity, between two tai imits. In the light 
(of the theoretical v ws now » we must conclude that the matter ua- 
: for heat, or of different sized groups of 
Report of Rey. Baden Powell, MA. PRS, for 1854, on Radiant Heat, 
1@ in the Smithsonian Report for 1859, p. 368.) 
