333 On the Ohjeclions to Mr. Prevosl's Explanation [May, 



Article V. 



Observations on the Oljeciions that have been made to Mr. Prevosd 

 'Explairation of the Effect produced by a Mass of Ice on a 

 Thermometer in the Focus of a Refecting Mirror. By Mr. 

 Richard Davenport. 



(To Dr. Thomson.) 

 srti. 



Being on a visit in Edinburgh, I took advantage of the oppor- 

 tunity it gave me of attending the lectures of the Professor of 

 Chemistry, and witnessing a series of admirably conducted experi- 

 ments. In a lecture on caloric an experiment was made with a 

 conical tube of polished metal, the result of which has been consi- 

 dered as tending to prove the actual radiation of positive cold. On 

 considering the phenomenon here presented, it appeared to me that 

 it might be accounted for exactly on the principle on which an 

 apparent radiation of cold from a body of ice in the focus of a 

 concave metallic mirror, on a thermometer in the focus of a similar 

 mirror set opposite to it, has been explained ; which explanation I 

 had considered as satisfactory, and I had thought was generally 

 admitted. However, on looking into Mr. Murray's Elements of 

 Chemistry, I found he objected to Mr. Prevost's explanation, and 

 considered the phenomenon as not yet accounted for. I found also 

 in the New Edinburgh Encyclopaedia, Jklr. Murray's objections 

 stated and admitted, as objections unanswered, and the whole 

 question left in an undecided state. 



Now as it appears to me that the phenomena both of the mirrors 

 and the cone may be satisfactorily explained without supposing the 

 actual existence and radiation of cold ; and as the authorities 1 have 

 mentioned are much looked up to ; and as they leave an important 

 question in philosophy in a state of indecision ; if that explanation 

 which 1 have said appears to my mind satisfactory, should on 

 perusal appear so to you, you will perhaps not object to employ a 

 page or two in the Annah of Philosophy by the admission of the 

 following, sketch. I am, Sir, 



Your obedient humble servant, 



KicuARD Davenport. 



It is well known that if a body be placed in the principal focus of 

 a concave purabolic reflecting metallic disc, and a hot body in the 

 corresponding focus of a similar reflector standing at a distance 

 from, but duly opposite to, the first, the radiating caloric of the 

 hot body is reflected and condensed upon the body in the focus of 

 the first reflector ; proving that radiating caloric is reflected in like 

 mauuer with rays of light. But if a comparatively cold body is 



