On the different apparent Magnitudes of the same Oljects, 1 63 



coal supplies the heat requisite to this purpose, whilst the 

 other literally undergoes no con^bustion at all, but is trans- 

 ferred unaltered from the carlwn to the iron, or whatever 

 substance is employed; that is, part of the rei/inous fluid 

 which charcoal as a combustible contains., is expended in the 

 production of heat and gas, whilst the other part unites with 

 the iron (if such was employed) of the oxide, and renders 

 it combustible. The metal is rendered combustible at the 

 expense of the combustibility of the charcoal; and the reason 

 that the iron does not undergo combustion when rendered 

 combustible, is owing to the well-known chemical fact, that 

 substances alter in their propertiesby different combinations. 

 In this case we find that the resinous fluid (the principle of 

 combustibility) becomes more fixed in the fire by uniting 

 with the particles of iron, &c., than it was when united with 

 the particles of carbon. It appears, therefore, that the re- 

 sinous fluid (the^rinciple of combustibility) is more or less 

 fixed in the' fire, according to its combination with difl^erent 

 substances ; and that it is owing to this that to burn one 

 combustible one degree of heat is required, and another an- 

 other. 



XXXI. On the Cause of the different apparent Magnitudes 

 of the same Objects seen under dfferent Circumstances, 

 By Ez. Walker, Esq. 



To Mr, Tilloch, 



SIR, 



At the conclusion of a paper which was printed in the 

 Philosophical Magazine for last October, I mentioned that 

 the apparent magnitudes of all objects are variable, as well as 

 those of the sun and moon. 



This, however, is not a new discovery, for M. le Cat 

 mentions it in several parts of his Physical Essay on the 

 Senses, printed in the year 1739. This philosopher says, 

 p. 234, " I looked through the glass of a casement, at a 

 very remote country-seat, which appeared to me sufficiently 

 large. I afterwards fixed my eyes on the glass itself; and it 



L 2 seemed 



