REPORT ON RADIANT HEAT. 
299 
is accurately known, may be put with regard to the modifica¬ 
tions (if any) which radiant heat may undergo, in passing 
through small apertures ; this will again be connected with the 
interceptive power of net-work. A very curious and delicate 
subject of inquiry is the repulsion exerted between heated 
bodies at sensible distances, of which a short notice is given in 
the Quarterly Journal of Science, xxxix. 164. 
The reflexion of heat has been little examined, except in the 
single case of its concentration by spherical reflectors ; and 
here (according to Leslie,) it is not brought to the same focus 
as light: this requires examination, as well as the simpler case 
of plane surfaces, and the proportion of heat reflected at dif¬ 
ferent incidences. There will probably in all cases be a very 
large deduction to be made for the heat acquired by the reflector 
and radiated again. 
But another class of such questions yet remains in connexion 
with that fundamental point which was the object of my first 
inquiries. The conclusion from my experiments, viz. that 
luminous hot bodies are sending forth at the same time two di¬ 
stinct species of heat distinguished by different properties, is 
the unavoidable conclusion from the experiments, depending 
on the mathematical truth, that if a ratio be altered by the 
addition or subtraction of quantities from its terms, the quanti¬ 
ties added or subtracted must be in a different ratio from the 
original one. I here repeat this because the nature of the 
reasoning has not been perceived by some persons. This con¬ 
clusion undoubtedly introduced a complexity into the view we 
must take of the phenomena; whereas if we were at liberty to 
adopt the simpler theory of De la Roche and others, many of 
the apparent anomalies would be reconciled. Hence the veri¬ 
fication of my results becomes a point of considerable import¬ 
ance. If any experimenter with more accurate apparatus shall 
succeed in showing them to be erroneous, he will achieve an 
important step towards simplifying the theory. In this instance 
again the improvement of the thermometer is a primary re¬ 
quisite. 
I may here mention that I have recently had a more delicate 
apparatus made, with which I have repeated my former experi¬ 
ments, still with the same result; it consists of two thermo¬ 
meters mounted together as before described. They were 
contrived for me by Mr. Cary, so as to have very large de¬ 
grees for a small part of the scale a little above ordinary tem¬ 
perature. 1° Fahr. occupies about half an inch ; but the 
bulbs are large, which is unfavourable to the rapid communica¬ 
tion of the effect. These experiments are of a very tedious 
