402 Mr. Powell on Light and Heat. [June, 



Argand lamp. Instrument stationary at, 



Screened. Exposed. 



Indian ink 9 12 



Silk 1 8 



(2.) In an account given by Dr. Wollaston of his celebrated 

 researches on the chemical effects of solar hght (Nicholson's 

 Journal, 8vo. vol. viii. p. 293), after showing that the green 

 colour which is communicated to guaiacum by the violet rays, is 

 removed on exposure to the red ; and that the same effect is 

 produced by the apphcation of hot metal by conduction, Dr. W. 

 makes the following remark : — 



" The last experiment may possibly appear to have been 

 unnecessary ; but until it is explained why the heat that accom- 

 panies the sun's rays penetrates the substance of transparent or 

 semitransparent bodies, while the radiant heat from a fire has 

 scarcely power to enter even the most transparent, but princi- 

 pally scorches the surface, and is thence slowly conducted into 

 the interior parts : no degree of caution upon a subject so imper- 

 fectly understood should be deemed superfluous." P. 297. 



I have quoted this instance of the distinguished author's judi- 

 cious and well known caution as a contrast to many passages 

 which might be found in the writings of some of our most 

 eminent philosophers. The confessedly imperfect state of our 

 knowledge upon these subjects must show the importance of 

 every step we can with caution and certainty take towards the 

 elucidation of them. Instances are not wanting in the produc- 

 tions of very distinguished men which exhibit a great vagueness 

 and obscurity of ideas on these points, an evil which has proba- 

 bly been much increased by the adoption of theoretical views 

 respecting " calorific rays," " luminous caloric," " non-lumi- 

 nous Ught," &c. 



(3.) As bearing upon an important part of the subject, I am 

 led to notice the following theoretical view of the mode in which 

 a glass screen acts, on the supposition of an actual radiation of 



heat through it, given by Biot : (Traite de Phys. iv. 636) 



" Si I'air a travers lequel la transmission s'opere, ahsorbait une 

 portion sensible de ce calorique, et lui laissait un passage d'au- 

 tant plus libre, qu'il emanerait d'un corps plus chaud. On verra 

 tout-a-l'heure que cet ejjet a lieu pour les lames deverre, quandon 



les interpose dans le courant calorifique ;" Sec. 



If this were the case, it is difficult to conceive how a thin 

 plate of glass should cause a less diminution of effect than a 

 thick piece, for the thin glass would certainly abstract much less 

 heat, and the more heated the glass became, the less heat it 

 would absorb, and therefore transmit less, both which we know 

 are contrary to the fact. 



(4.) In reference to the history of investigations respecting 



