48 
REPULSION PRODUCED BETWEEN BODIES BY HEAT. 
perimenls, though simple in principle, cer- 
tainly require some care; but with all pre- 
cautions, and after the most careful consi- 
deration of all causes which can have tended 
to produce or etfect the result, it appears to 
me that the separation of the glasses through 
the extremely small, but finite and known 
-spaces, whose changes are indicted by the de- 
gradation of the tints, can only be due to 
the real action of a reptilsive power, pro- 
duced or excited between the surfaces of 
the glasses by the action of heat. 
There are many questions relating to the 
nature and properties of this repuls i’^e 
power, which are immediately suggested, and 
some of which appear capable of solution by 
variations of the same method. 
The distance at which the repulsive 
power can act, is shown by these experiments, 
to extend beyond that at which the most 
extreme visible order of Newton’s tints is 
formed. But I have also repeated the ex- 
periment successfully w'ith the colours formed 
under the base ofa prism placed upon a lens 
of very small convexity ; and according to the 
analysis of these colours given by Sir J . 
Uerschel, (on Light, 64,) the distance is 
here about the l,IU0th of an inch. 
Beyond these very small distances, other 
methods must be resorted to. But the cer- 
tainty of the result within these limits per- 
haps confirms its probability at greater 
distances, as inferred by Fresnel and Saigey. 
I tried several experiments on the etfect s 
of difterent sorts of surfaces, from which I 
conceive, though w e may infer that cceteris, 
paribus, the better radiating power of the 
surface increases the etfect; yet there are 
other circumstances w'hich atfect the result 
more powerfully, and these seem to be, in 
general, lohatever may tend to the more 
rapid communication of heat. 
This is still more conspicuous w'hen the 
rings are formed in a thin plate of water be- 
tween the lenses. The etfect is here even 
greater than in air, and w^e may presume, in- 
dependent of radiation . 
There are several subordinate circumstances 
attending these results wdiich are deserving 
of notice. When the lenses are in close 
contact, there is, in all cases, a considerable 
attraction opposed to tiie repulsive power. 
If the central black be. formed, it requires a 
very considerable intensity of heat to over- 
come the attraction, which at that minute 
distance is extremely powerful. 
When the heat is removed the colours 
return, and the rings are gradually restored 
to the same character as they had at first. 
This is more remarkable when simple plates 
of glass are employed as before described. 
When the heat has restored the bent glass 
to a plane figure, on its removal the rings 
return, and consequently, the glass is again 
bent without any fresh pressure, though the 
force originally applied to produce the curva- 
ture was very considerable ; this is probably 
owing, in a great measure, to atmospheric 
pressure. In this case, however, the colours 
w'ill only jeturn up to a certain point, gene- 
rally not higher than the beginning of the 
first order. 
When two glasses are pressed together 
there is a repulsion to be overcome, evinced 
by the force which it is necessary to apply, 
and in general, it is evident, that if a plate 
resting on another be bent by pressure, as in 
these experiments, the influence of heat in 
restoring it to a plane form will be opposed 
both by the attraction at the centre, wdiich 
tends to prevent that part from being raised, 
and by the repulsion towards the exterior 
parts, which t-nds to prevent them from 
being depressed. When the curvature begins 
to change, therefore, there is somewdiere 
between them a neutral or nodal point w hose 
position does not change ; this point may be 
very near, or even in the centre, when the 
attraction is very strong there. A remarkable 
instance of this occurs when the first black of 
the scale is formed between glass plates, and 
heat carefully applied exactly over the central 
point of the black space ; in this case, when 
the black space is a ^ inch or more in diameter, 
I have often continued the application of the 
strongest heat for a great length of time be- 
fore any separation could be effected, when 
at length it has taken place with a sudden 
force and an audible click. Sometimes the 
black spot has continued unaltered until the 
glasses have cracked, wdien the fragments 
have still continued to adhere powerfully : 
meanwdiile the outer rings have continued 
gradually enlarging. 
In the foregoing statement, I have observed, 
that in using plane glasses, it was necessary 
to allow for the etfect of warping. But 
there are certain considerations which show 
that that precaution is unnecessary. For, 
according to the beautiful experiments of 
Sir D. llrewster on the progress of heat 
through glass, as evinced by its action on 
polarized light, it appears distinctly, that 
the change of structure (if I may so speak) 
in the molecules of the glass is produced 
at the same instant, on both sides of the 
plate : so that the etfect of ivarping cannot 
take place. This is rendered evident to the 
eye, by the symmetrical arrangement of the 
luminous bands, from the first moment of the 
application of heat, on each side of the dark 
central band, which occupies the neutral 
line along the middle of the thickness of the 
glass. 
When two plates of glass are laid upon 
one another there is a certain resistance or 
repulsion which may be overcome by pressure. 
We can press them together till attraction 
takes place. On removing the pressure they 
remain adhering. If we press them more 
they are brought closer, and produce the 
colours of thin plates. We may thus produce 
successively and given tint, and on removing 
the pressure that tint will remain, or the 
glasses continue in the same position to w Inch 
they have been brought. 
Tliis seems to show that the attraction and 
repulsion are in exact equilibrio at all distan- 
ces, (w ithin this range,) and this may hold good 
with any law, provided the law be the same 
for attraction as for repuision. 
On the application of heat a greater inten- 
sity of repulsion is excited ; if w e could ascer- 
tain the law' of its increase with the distance 
