1824.] Mr. Potoell on Terrestrial Light and Heat. 187 



meter having its bulbs variously coated. I must, however, 

 for the present retrain from going into further particulars, 

 except to make an observation on the mode of experimenting 

 with two thermometers and a glass screen. 



(13.) It may possibly be said that the screen might exert 

 a cooling influence, which would, from the nature of the coating, 

 produce a greater fall in the white than in the black thermo- 

 meter ; and thus a difference in the ratio of the two effects 

 might result. Any such effect would, however, be fully coun- 

 teracted by the sizes of the bulbs employed. The effects of the 

 coatinos, supposing them laid on nearly as in the former expe- 

 riments, (See Annals, Aug. § 38.) would be, white : black : : 

 100 : 89, and the diameters of the bulbs were respectively 0*6 

 and 0'55 inch. Hence we have the ratio very nearly, white : 

 black :: 100 : 90, from which it is obvious that any cooling 

 effect would be completely counterbalanced. 



But independently of any such considerations in most of 

 these experiments, more was to be apprehended from the 

 heating, than from the cooling power of the screen. And 

 though in experiments with the differential thermometer, where 

 the bulbs were nearly equal, the latter effect may have sensibly 

 interfered, yet the former probably much more than counter- 

 balanced it ; and in allowing for it regard was had to this con- 

 sideration. 



(14.) In speaking of the radiation from red hot iron (above 

 5; 4.) I alluded to the heating effect of the light which it 

 emits. Relative to this point so intimately connected with the 

 subject here proposed, 1 may be permitted to make a further 

 remark. On this point M. Biot has brought forward some 

 observations which require attention. When describing De la 

 Roche's experiments, and speaking of those in which he em- 

 ployed hot metal, he observes in a note: 



" On pourrait au premier abord, ctre etonne que De la Roche 

 ait appuye en partie sa proposition sur des experiences faites 

 avec un lingot de cuivre amene a la temperature de 960° ; car a 

 cette temperature il devait etre en incandescence ; mais il s'est 

 assure que la portion de lumiere qui s'en degageait alors ne 

 pouvait, comme lumicre echauiiante, influer sur l'ascension du 

 tlieimomctre que clans une proportion mnniment faible. Car, 

 en comparant les distances auxqueiles les memes earacteres 

 d'impression pouvaientetre ius a l'aide de cette lumicre, et avec 

 celle d'une simple bougie, il a trouve qu'on ne pouvait certaine- 

 ment paslui attnbuer -tt, de l'ettet total." — (Traite de Phys. iv. 

 note, p. 642.) 



As I have already remarked, it seems scarcely allowable to 

 infer that the heating must accord with the illuminating power 

 of light from different source* ; indeed, in this respect, it has 

 long appeared to me more than probable that differences to a 



