1821.] Mathematical Principles of Chemical Philosophy, 85 



tendency to dilate itself, as before, and at the same time its 

 quantity will be diminished by the attractive force of the matter 

 in its vicinity ; the atmosphere can, therefore, only become 

 quiescent, when the requisite equilibrium of the two former cases 

 is attained, and the surrounding bodies have arrived at the tem- 

 perature of the parts of the atmosphere in which they are placed. 

 During this operation, a current of heat must flow from the 

 heated to the cooler mass, which, at its several distances from 

 the heated body, will have the same density as the equidistant 

 strata of the atmosphere. Hence the colder the body, which is 

 absorbing the heat, is made, the more rapidly will the other lose 

 its heat. Also of bodies equally cold, those which have the 

 greatest capacities for heat cool the other more than those which 

 have less capacity. 



These three phenomena explain many of the primary proper- 

 ties of radiant heat. When bodies of equal magnitude and 

 figure a^e heated to the same degree, then brought into a cooler 

 medium, those which have^ the greatest attraction for heat, by 

 prop. 6, cor. will be surrounded by atmospheres of the greatest 

 density, and consequently at equal distances from the surface, 

 will cause the greatest elevation of the thermometer ; if black, 

 and generally dark coloured bodies be supposed to attract heat 

 more than those that have a lighter colour, this will exactly 

 coincide with the observed phenomena ; for if surfaces equally 

 heated, and of equal magnitude and form, be one covered with 

 lamp-black, another with red-lead, and another with tin-plate, at 

 equal distances from these surfaces, the effects produced upon 

 the thermometer will be nearly in the proportion of the numbers 

 100, 80, and 12 (Davy's Elements, p. 209) ; which is explained 

 by supposing the attraction for heat of the lamp-black to be 

 greater than that of red-lead, and this -'greater than that of tin- 

 plate. It appears from prop. 6, cor. and prop. 11, that this will 

 not be the order of their several capacities for heat ; conse- 

 quently, if equal and similar masses of the same matter be equally 

 heated, and have their surfaces differently coloured, those 

 which have the darkest colour will part with their heat in less 

 time than those which are lighter coloured, or polished. 



The cause of the reflection of heat may 

 now be easily explained. Suppose A, fig. 1, 



to be a heated body placed in a cold me- /^® 



dium, its calorific atmosphere tends to en- i,;:'^^._.^:"l{...*S 

 large its dimensions, or to dilate itself, moving ^^ '* 



in right fines from the centre of A. If 

 it be placed in the focus of a parabofic mirror B C, which is of 

 such a nature as to have feeble attraction for caloric, after 

 impinging upon this surface, it will rebound, or be reflected ; and 

 since the angle of incidence is equal to the angle of reflection, 

 by the property of the parabola, these incident calorific x^jn 



f2 a 



