A PLATINUM SURFACE AT HIGH TEMPERATURES. 515 



(2.) LEPINAY and NICATI* have shown that the intensity of a light is not the 

 same when measured by the distinctness with which the details of an 

 object can be seen as when measured by the apparent brightness of a 

 surface. The form of photometer used is sufficient indication that it is 

 the latter method which has been adopted for the experiments recorded 

 here. 



In 1879 ViOLLEt published a research on much the same subject as is here dealt 

 with. His results and mine though in substantial agreement as to the intrinsic 

 brilliancy of platinum at its melting point, differ considerably at lower temperatures. 

 In the paper alluded to no information is given as to the methods or instruments 

 used, it is therefore impossible to suggest to what the divergence may be due. The 

 author sums up his results by giving for the relation between the temperature, t, and 

 the light emitted, I, the formula 



Log I = 8"244929 + 0'011475 - 0'00000297t 2 . 



This expression by a modification of the constants could doubtless be made to agree 

 with the present results ; but the formula given below, owing to its simplicity, will 

 be found of more practical value. 



If t denote the temperature in degrees centigrade, and b the intrinsic brilliancy of 

 the surface in candle-power per square centimetre, the relation may be expressed as 



(t 400) = 889-6 V 



The temperature calculated by this formula ^ will be found in column 3 of Table XVIII. 

 The crosses on fig. 13 represent the intrinsic brilliancy as calculated. The circles 

 show the values obtained experimentally. Where the formula is used over so wide 

 a range of temperature the agreement for the lower values is but approximate ; but 

 this expression will be found to give very correct values of b when used for a limited 

 interval above and below any temperature for which the constant has been 

 determined. 



Finally, before bringing this paper to a close, I desire to express my deep indebted- 



* ' Annales de Chimie et de Physique,' .1883, vol. 30, p. 145. 



t ' Comptes Rendus,' vol. 88, 1879, p. 171. 



J When the platinum is observed in a dark room, the first grey light becomes visible at a tempera- 

 ture of 417 C. It is as the limiting temperature, so far as visible radiation is concerned, that 400 C. 

 is used in this formula. 



I have recently made a number of observations on the intrinsic brilliancy of the crater of the 

 electric arc. As these results refer to a study at present far from complete, it will be sufficient to state 

 here that the intrinsic brilliancy is about 11,000 candle-power per square centimetre. Inserting this 

 value for b in the formula given above, we obtain 3830 as the temperature of the crater. This fact may 

 be taken as a confirmation of the formula for temperatures above the melting point of platinum, 

 though I do not think that it would be wise to attach much importance to a coincidence of this kind. 



3 U 2 



