254 Prof. E.Wiedemann on the 



The number 4*7 obtained for the total energy is taken as 

 the basis of all the following calculations. 



21. From these numbers certain farther conclusions can be 

 obtained. 



We will first calculate the quantity of energy radiated per 

 second from 1 gramme and from one atom of platinum. 



The above quantity of energy, 4*7 calories, is emitted by a 

 layer of unit surface, which has such a thickness that the rays 

 coming from the deepest points still perceptibly issue from the 

 surface. The different layers contribute in different degrees 

 to the radiation. For the sake of simplicity we will assume 

 that all the layers do this equally. If d is the thickness of the 

 radiating layer, s the specific gravity of the radiating sub- 

 stance, then the weight of the radiating layer under the unit 

 surface is y = sd. For d I put the approximate value d^l0~ 5 

 cm., corresponding to the numbers of Herr Wien *. Further, 

 for platinum, s = 21\5 ; then the weight of the radiating layer 

 for platinum is 2*15 x 10~ 4 gr. One gramme of platinum 

 consequently emits in one second a quantity of energy, 

 2-2 xlO 4 . 



The total emission-coefficient of 1 gramme of platinum at 

 the temperature used of about 1000° C. is therefore 



S = 2*2 x 10 4 gr. calories per second. 



But further, according to the calculations of van der 

 Waalsfj the absolute weight of a single atom of hydrogen is 

 7*5 x 10~ 23 gr., the atomic weight of platinum 194 ; conse- 

 quently the weight of an atom of platinum is, in round num- 

 bers, 15 X 10~' 21 gr. Consequently an atom of platinum at the 

 temperature of about 1000° C, assumed for the purposes of 

 our calculations as that of the platinum wire, emits 2'2 x 10 4 X 

 15 x 10~ 21 = 3'3 x 10 -16 gr. calories per second. 



We will further determine the relationship existing between 

 the quantity of heat emitted at 1000° C. and the quantity 



well as that of Herr Turalirz, " Calculation of the Mechanical Equivalent 

 of Light from the experiments of H. J. Thomsen " (Ibid. pp. 1625-1632. 

 20/12. 1888). This gives the radiation of glowing platinum per square 

 centimetre, and a comparison of the brightness of the glowing platinum 

 with the acetate lamp. My values agree with his also as to order of 

 magnitude. The differences may be explained by the fact that different 

 wires behave differently; and that these gentlemen, whose object was 

 the exact determination of this magnitude, have introduced a number of 

 corrections which I did not consider necessary. As I was only concerned 

 to determine the order of magnitude of the several quantities, nothing 

 essential in the conclusions is altered by the introduction of the values of 

 other observers instead of mine. 



* Wied. Ann. xxxv. p. 57 (1888). 



f Cf. R. Ruhlmann, Mech. Wdrmetheorie, ii. p. 247 (1885). 



