Progress of Foreign Science. 145 



fixed temperature q less than b, and greater than a ; now, the 

 value of Q is given by the following equation. 



b — a 

 Q-a= . 



1 + e"* 

 By e we denote the depth of the solid, or the distance between 

 the two extreme planes ; k and h are specific coefficients, that 

 is to say, peculiar to the substance of which the solid is formed. 

 These coefficients must be determined by observations. The 

 first k measures the facility with which the heat travels in this 

 substance, from one particle to another. The second h mea- 

 sures the facility with which the heat is dissipated by the sur- 

 face. This effect includes two modes of cooling, namely, that 

 due to radiation, and that occasioned by the contact of the air. 

 If, for different substances, the coefficient h is the same, or 

 little different, and if the temperatures b and a, and the depth 

 e, are the same, the difference q — a will be greater when the 

 conductibility is greater. 



2. Suppose that ahomogeneous solid mass is terminated by an 

 infinite plane, and that the depth of the mass below this plane 

 is extremely great. Let us assign to every point of the mass a 

 common initial temperature, b, and let the plane which termi- 

 nates it remain exposed to the air, kept at the fixed tempera- 

 ture zero. The initial temperature b decreases slowly, in pro- 

 portion as the heat is dissipated in the air. After a given time 

 marked t, the temperature q of the surface has a certain value 

 expressed in a function of t, and of the specific coefficients h 

 and'^, which regulate the motion of the heat, in the substance 

 of which the solid is formed. It is required to know how far 

 this decreasing temperature q depends on the peculiar conduc- 

 tibility of the solid. M. Fourier gives an equation of some in- 

 tricacy, from which he infers that the temperaiure q of the sur- 

 face is so much greater, as the heat is more easily conducted 

 in the interior of the solid ; all other conditions being supposed 

 the same *. 



Under the head of Caloric maybe ranged phosphorescent 

 phenomena. A mixture of antimony and cream of tartar, ex- 

 posed to the action of fire in a close crucible, leaves a residuum 

 which is an alloy of potassium and antimony. This forms, in a 

 certain state, a very active pyrophorus. Tartar emetic in powder, 

 enclosed in a covered crucible, when ignited for a couple of 

 hours, presents after cooling a carbonaceous-looking mass, mara- 

 melated above like cauliflower, and interiorly radiated like an- 

 timony. This inass inflames suddenly in the air, while the revived 

 antimony runs together into globules of extreme brilliancy, 



* Ann.de Chimieet de Physique, xvi. 82. 



Vol. XIII. L 



