the radiant heating effects from terrestrial sources. 199 
ratio very nearly that which the effects of simple radiant heat 
would give. 
III. (25.) I have above adverted to all the sources of error 
which occur to me as likely to have affected these results ; and 
when these are taken into consideration, as well as the nature 
of the experiments and apparatus, the accordance which the 
different results exhibit, is perhaps as close as we can ex- 
pect ; and it appears that all the different sets of experiments 
agree in showing a very considerable difference in the ratio 
of the effects produced on a smooth black, and on an absorp- 
tive white surface, by that part of the radiant effect trans- 
mitted through glass, and by the total effect. If the total direct 
effect were the result of one simple agent, the intervention 
of the glass would, by intercepting some part of it, produce 
no c ,her alteration than a diminution of intensity ; the ratio 
of the two effects would remain unchanged. This distinction 
appears to me of some importance towards clearing our 
ideas respecting the nature of the phenomena, and thus afford- 
ing an answer to the question originally proposed in refe- 
rence to some theoretical views, which, though boasting the 
sanction of high authority, will be untenable if the validity of 
these results be admitted. 
(26.) The general conclusions from all these experiments 
may be thus recapitulated : 
1st. That part of the heating effect of a luminous hot body 
which is capable of being transmitted in the way of direct 
radiation through glass, affects bodies in proportion to their 
darkness of colour^ without reference to the texture of their 
surfaces. 
2d. That which is intercepted produces a greater effect in 
