190 Mr. Talbot on the Repulsive Power of Heat. 
glass. The plano-convex form of the particles is owing to 
the force with which they endeavour to recede from the lower 
glass, and their consequent pressure against the surface of the 
upper one. I think this experiment is a satisfactory argu- 
ment in favour of the repulsive power of heat, and I believe 
it has not been hitherto described. 
Experiment 2.—On the Vaporization of Arsenic. 
When a particle of arsenic is sublimed between two plates 
of glass, it forms nebulous patches, considerably resembling 
those formed by sulphur in the preceding experiment. But 
the microscope detects a great difference. Instead of a glo- 
bular or semiglobular form, the particles of arsenic are cry- 
stallized. ‘The minuteness of some of the crystals almost ex- 
ceeds calculation. I would suggest the employment of this 
method to detect the presence of arsenic in minute quantities 
of matter. The difficulty of demonstrating its presence with 
sufficient certainty is shown by the number of chemical essays 
that have been written on the subject, while a particle of the 
size of a pin’s head is amply sufficient to display this micro- 
scropic crystallization; and the form of the crystals being di- 
stinct and definite, the observer can soon make himself ac- 
quainted with their figure, so as to run little risk of mistaking 
any other substance for them. 
Note on Radiant Heat. 
M. Melloni says (in the Number of this Journal for Decem- 
ber last, vol. vii. p. 475,) that 
‘** For a long time the immediate transmission of terrestrial 
radiant heat by transparent substances, both solid and liquid, 
has been denied; and the opinion has become prevalent that 
we see in experiments of this kind only an effect of the heat 
absorbed by the body submitted to the calorific radiation.” 
This “ prevalent opinion” he has shown to be erroneous, 
but by experiments which are too delicate to be repeated with 
facility. 
As a popular illustration of the fact, therefore, seems to 
be wanted, I subjoin the following rude but convincing expe- 
riment. 
Let a poker be heated bright red hot, and having thrown 
open a window, approach the poker quickly to the outs¢de of 
a pane, and the hand to the inside. A strong heat is felt at 
the instant, which ceases as soon as the poker is withdrawn, 
and may be again renewed, and made to cease as quickly as 
before. Now, everybody knows that if a piece of glass is so 
much warmed as to convey this impression of heat to the 
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