4A M. MELLONI ON THE FREE TRANSMISSION 
itis then exposed to a heat twice as strong, and therefore exhibits a far 
greater effect of conduction. Hence it follows that when we deduct from 
the observation furnished by the transparent glass the observation fur- 
nished by the glass blackened, the result obtained will be lower than the 
true temperature of the rays transmitted freely. But the error will not 
be the same in all cases. Being of no account when boiling water is em- 
ployed, it will increase in proportion as the temperature of the source is 
raised. The measures of the free radiations which suffer the greatest 
diminution will be those furnished by the highest temperatures. Hence 
it is evident that this latter cause of error in the measure of the imme- 
diate irradiation, instead of invalidating the law of Delaroche, serves 
only to give it greater certainty. We are therefore justified in saying, 
as we have said, that the want of exactness in the method has no 
influence whatsoever on the truth of the law which it has served to 
establish. 
To Delaroche we are also indebted for a discovery, no less important 
than the foregoing, relative to the amount of loss sustained by the same 
rays of heat in passing successively through two squares of glass. But 
I abstain, for the present, from entering into any detail on this subject, 
as I shall have occasion to speak of it hereafter *. 
None of those whose labours we have been thus briefly noticing has 
thought of making an exact comparison between the transmissions of 
caloric rays through screens of different kinds; and, if we except the 
experiments of M. Prevost and those of Herschel, from which no con- 
sequence can be deduced, all the others were confined to the single pur- 
pose of.ascertaining the law of transmission through glass only. Neither 
has sufficient attention been given to the influence of the state of the 
* T must not omit to mention that, notwithstanding the results obtained by 
Delaroche, some most eminent philosophers (and of these it will be sufficient to 
name Laplace and Brewster) continued to deny the immediate transmission of 
heat through transparent solid bodies. ‘Their principal objection was founded 
on an experiment of that author, from which it was inferred that a thick glass 
intercepted a greater quantity of radiant heat than a thin glass, though the for- 
mer was much more transparent. It was insisted that this cireumstance proved 
the presence and action of heat successively propagated from one surface to 
the other, and every elevation of temperature observed on the other side of the 
screen was assigned to the conductible caloric. This opinion can no longer be 
maintained in defiance of the results furnished by the application of the ther- 
momultiplier to this species of phenomena. It will be seen, further, that the 
calorific action through a transparent layer is instantaneous, and that the time 
necessary for the instrument to mark its total effect is the same, whatever be the 
quality or thickness of the screens. Let the direct rays from an unvarying 
source of heat be received on the thermoelectric pile; let them be first made to 
pass through any diaphanous screen of one hundred millimetres in thickness: 
the index of the galvanometer sets itself in motion from the instant when the 
communications are established, and stops after having described an arc of 
greater or less extent in an wnvarying interval, which, with my apparatus, I 
find to be ninety seconds. 
