366 PROFESSOR TYNDALL’S CONTRIBUTIONS TO MOLECULAR PHYSICS. 
For the same spiral lowered to moderate redness, the transmission was 
83*6 per cent. 
Nothing was changed in these experiments but the heat of the spiral ; the direction 
of the rays, and the size of the radiating body, remained throughout the same ; still we 
find a gradually augmenting opacity on the part of the rock-salt as the temperature of 
the source is lowered. There cannot, I think, be a doubt that MM. De la Provostaye 
and Desains are right jn their conclusion that rock-salt acts differently on different calo- 
rific rays, and is not, as Melloni supposed, equally transparent to all. For the heat of 
the hydrogen-flame it is more opaque than for that of the moderately red spiral. 
§14. 
This memoir ought perhaps to end here ; I would, however, ask permission to make 
a few additional remarks on a subject which was briefly touched upon towards the con- 
clusion of the first of this series of memoirs. I make these remarks with diffidence, for 
I have reason to know that authorities for whom I entertain the highest respect do not 
share my views regarding the connexion which subsists between the radiation and con- 
duction of heat. 
Let us suppose heat to be communicated to the superficial stratum of the molecules 
of any body ; say, the molecules at the extremity of a metal bar. They vibrate, and 
the motion communicated by them to the external ether is dispatched in waves through 
space. The vibrating superficial molecules must also set in motion the ether within the 
body, and a portion of this motion will be transferred to the stratum of molecules next 
adjacent to the superficial ones, heat as a consequence appearing to penetrate the mass. 
But irrespective of the ether, the molecules of the body occupy positions which are deter- 
mined by their attractive and repulsive forces ; so that if any one molecule be forcibly 
moved from its position of equilibrium, it "will of necessity disturb its neighbours. In 
a system of molecules so related, it is manifest that motion could be transmitted inde- 
pendently of the ether which surrounds them. If we could imagine the ether entirely 
away, the motion that we call heat would still be propagated from molecule to molecule 
through such a body. Conduction would manifest itself, while radiation would be absent 
through want of a medium. 
In matter, however, as we have it, molecular motion is only in part transmitted imme- 
diately from molecule to molecule, being in part transmitted mediately by the ether. 
Now the quantity of motion transmitted by the ether to our second stratum of mole- 
cules cannot be the whole of that which the first or superficial stratum imparted to the 
ether. The ether must retard and indeed squander the internal molecular motion; 
and were the medium absent — were the cushion removed which interferes with the 
direct propagation of motion from molecule to molecule — conduction would be freer 
than at present ; the heat would penetrate further into the mass than when the ether 
intervenes. 
