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the radiant heating effects from terrestrial sources, 
present so simple a theory as that alluded to, but they apply 
very obviously to the explanation of many phenomena re- 
corded by various experimenters. 
(28.) The peculiar heat above spoken of, and which, for 
the sake of distinction and brevity, we may call “ trans- 
missible heat,"' is similar to that which acts in the solar rays, 
and which there constitutes the total effect. It is this kind of 
heat which has been employed as a principle of photometry, 
on the assumption that it is precisely proportional to the in- 
tensity of light. Within certain limits this may be the case ; 
but there are unquestionably circumstances under which the 
relation is very different ; such for example, as difference of 
colour in the light ; and in general it cannot be assumed to 
hold good in light from different sources. To show this, 
there is a remarkable instance in incandescent metal, which 
produces but very faintly illuminating- rays, yet its “ trans- 
missible heat'' is very considerable. I have repeatedly tried 
the experiment with a small ‘‘ photometer," having one bulb 
painted with indian ink and the other plain ; the bulbs 
being in a vertical line ; this instrument whether employed 
with or without its case, or a glass screen, always gave an 
effect of about 10“ in 30" at eight inches distance from a 
ball of iron heated to the brightest point in a common fire. 
(29.) In making these last experiments, the effect was 
always greater when the instrument was used without its 
>case, or a glass screen. This was no doubt in part owing to 
the greater action of the simple heat now admitted to the 
instrument on the coated, than on the plain bulb ; but it was 
also in part occasioned by the circumstance, that the stem 
going to the upper bulb passes in contact with the lower, 
MDCCCXXV. D d 
