as exhibited in its propagation along plates of glass, 113 
immediately above it. In this way, I have obtained a light as 
brilliant as that which is reflected from silver. The internal 
reflections may be removed by interposing a film of oil be- 
tween each of the plates, so as to rise above that part of the 
plate where the tint is to be examined. 
If the instrument is properly constructed, with these pre- 
cautions, I have no hesitation in saying, that it will distinctly 
mark a difference of temperature equal to i° of Fahrenheit's 
thermometer.* 
I have thus endeavoured to give a brief view of the nume- 
rous experiments which have led to the general results un- 
folded in the preceding enquiry. The length to which this 
paper has extended, has prevented me from describing many 
phenomena, and detailing many experiments, which, though 
interesting in themselves, did not appear absolutely necessary 
to the establishment of general views. 
Had I included in the demonstration of every proposition, 
the various experimental proofs which I had actually obtained, 
this Paper would have swelled to a size which would have 
rendered it unfit for the consideration of the Royal Society ; 
I have, therefore, selected such experiments as appeared most 
striking, and have left the detail of the rest, and the repre- 
sentation of many of the phenomena, for a separate work 
which I propose to publish on the subject.^ 
* This thermometer possesses advantages peculiar to itself, in enabling us to mea- 
sure the intensity of the heat produced by the friction of any two substances whatever. 
When glass is one of the substances, the method of employing the instrument is ob- 
vious. When any other substance is used, it must be fixed, without cement, to the 
lower edge of one or more plates of glass, so that its rubbing surface may be as near 
as possible to the edge of the glass. 
f There is one practical result of the preceding experiments, which deserves parti- 
cular notice. All articles made of glass, whether they are intended for scientific or 
MDCCCXVI. O 
