J' M. MELLOXI OX THE FREE TRANSMISSION 



it is 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 bj' the transparent glass the observation fur- 

 nished bj' 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 tmth of the law which it has served to 

 establish. 



To Delaroche we are also indebted for a discoveiy, 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 tlie present, from entering into any detail on this subject, 

 as I shall have occasion to sjieak 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 tlie state of the 



• I 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 quantitj- of radiant heat than a thin glass, though the foi'- 

 nier was much more transparent. It was insisted that this circumstance proved 

 the presence and action of heat successively propagated from one surface to 

 the other, and everj' 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- 

 momulfiplier to this species of phsnomena. It will be seen, further, that the 

 calorific action through a transparent layer is instantaneous, ond that the time 

 necessary for Mie instrument to maik its total effect is the same, whatever be the 

 quality or tliickness 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 histaiit when the 

 communications are established, and stops after having described an arc of 

 greater or less extent in an unvarying interval, which, with my apparatus, I 

 find to be ninety seconds. 



