Prof. Powell on the alleged Polarization of Heat. 207 



of heat, in which I stated, that, as far as I was aware at that 

 time, M. BerarcTs researches on the subject had not been pub- 

 lished. I have, however, recently been informed that I was 

 mistaken ; and being referred to the 3d volume of the Me- 

 moires de la Societe oVArcueil, (Paris, 1817,) I there found 

 the paper in question, entitled " Memoire sur les proprietes 

 des differentes especes de rayons qu'on peut separer au moyen 

 du prisme de la lumiere solaire, par M. J. E. Berard. Lu 

 a Imstitut le 21 de Dec. 181 2." 



I immediately examined the memoir with reference to the 

 question discussed in my former communication ; but, I must 

 confess, felt not a little disappointed at the very slight infor- 

 mation which the distinguished writer affords as to the parti- 

 cular points on which, it appears to me, so much of the force 

 and importance of the general result depends. 



The principal particulars which I can collect as bearing on 

 my questions are these : 



~\st, The thermometer employed was an air-thermometer, 

 containing a bubble of alcohol in the tube. 



2d, The apparatus, consisting of glasses like Malus's, had 

 the axis of revolution placed vertically. No mention is made 

 of any precautions in regard to the very important point of 

 guarding against irregular action of heat by screens, &c. 



3d, With regard to the radiation from hot metal, the source 

 was " une boule de cuivre de la grosseur d'un ceuf," which was 

 placed in the focus of a concave metallic reflector. It was 

 used in the first instance at a bright red heat, and in this case, 

 as well as in that of the solar rays, the results agree entirely 

 with what I have before stated. 



4>th, The author then proceeds to the case of simple heat 

 unaccompanied by light : this he seems to consider but of se- 

 condary importance, regarding it in connection with the views 

 inferred from De la Roche's experiments, but, after a few re- 

 marks, proceeds thus to describe his experiments : 



" -Pai attendu dans quelques experiences que la boule de 

 Cllivre ne fut plus visible dans robscurite ; etjeme suis as- 

 sure que les rayons quelle emettait dans ce cas pouvaicnt Are 

 aussi polarises." 



This meagre, not to say somewhat ambiguous statement, is 



NEW SERIES, vol.. v. NO. II. OCTOBEB IH'.i] . 



