Mechanics of Luminosity . 253 



Total Radiation from one Square Centimetre and one Gramme 



Platinum. 



20. In the determination made for the purpose of evaluating 

 the amyl-acetate lamp in absolute measure, the distance 

 between the slit and the wire was 44 millim. This is made 

 up of the distance of the slit from the edge of the glass tube 

 and the radius of the tube. The distance of the amyl-acetate 

 lamp from the slit was 216 millim. The length of the wire 

 was 7*6, its thickness 0*26 millim. Consequently the radia- 

 ting surface = 2irrl = 0*6205 square centim. 



If the current-strength (in amperes) is i, the resistance for 

 current-strength equal to w x (in ohms II), the resistance at 

 15° w 15 , the energy radiated by the whole wire F = 0"24 wi 2 

 (in cm. g. sec. cal.), the readings a on the photometer for the 

 part of the spectrum close to the sodium-line, the brightness 

 J measured by cotan 2 a, the energy radiated by the unit sur- 

 face E = G-{-0, then the following (amongst other) data were 

 obtained : — 



w? 15 = 0-23<m, 1^ = 0-75712, Wi/w l5 = 3'17 ; 

 i=4'00, F = 2-91, E = 4-68 ; 



a = 36° 30', J= cotan 2 « =1-827. 



A square centimetre of the surface of a platinum wire heated 

 to about 1000° C. consequently radiates about 4*7 gr. calories 

 in a second. 



The value of E agrees in order of magnitude with that of 

 H. Bottomley* for the highest temperature used by him ; 

 mine is higher. This is explained by the fact that the tem- 

 perature used by me is higher than his t- 



the above-mentioned dependence over a considerable region, then in the 

 first place no luminescence-phenomena must occur with the body in 

 question, and in the second place the molecules and atoms which with their 

 sether envelopes build up the body must undergo no changes. But such 

 a change certainly takes place in all bodies which show a great change in 

 specific heat upon increase of temperature. Since the specific heats in 

 the liquid and gaseous and in the liquid and solid conditions exhibit like 

 coefficients of change, so far at least as present investigations extend, the 

 quantity of heat corresponding to the change in specific heat can only 

 result from intramolecular work. But anything that brings about an 

 increased porosity of the molecular condition, or an increased intramole- 

 cular activity, must modify the emission and absorption of light in a way 

 not immediately determinable. 



* Phil. Trans, cxviii. p. 429 (1887). 



t It is only after completion of the present investigation that I have 

 become acquainted with the work of Messrs. O. Tumlirz and A. Krug, 

 '•On the Energy of the Radiation of Heat at a White Heat" {Sitzunqsber. 

 d. Wien. Akad. xcvii. pp. 1521-1529, 13/12, 1880), which follows a pre- 

 vious paper by one of the authors {Wien. Ber. xcvi. p. 1007, 1888); as 



Phil. Mag. S. 5. Vol. 28. No. 173. Oct. 1889. U 



