Research on some Standards of Light. 



493 



Fig. 8. — " Normal " conditions : total mass of platinum, 345 grammes ; superficial area, 17 sq. 

 cm. ; diameter of aperture in the cover of the furnace, 1'6 cm. ; intensity of the light at 

 the temperature of solidification, 1*002 (the error of this determination is therefore 0'002). 



O tO 20 30 4Q 6® 60 70 80 90 100 110 120 150 I4O/5Q/60 



Time in Seconds. 



is due to the fact that the heat lost at the surface is at first supplied 

 by conduction from the lower layers still substantially at the tem- 

 perature of solidification, and it is not until the corresponding tem- 

 perature gradient is established throughout the entire mass that the 

 normal rate of cooling is shown by the light given off from the 

 surface. 



Curves similar to the above, but referring to the case of molten 

 silver, were shown by Violle in 1881.* 



A second form of curve is possible (see figs. 9 and 11). This shape 

 is obtained when the metal has been heated very slightly above its 



* ' Annales de Chimie et de Physique,' ser. 6, vol. 3, p. 373, 1884 ; also Confer- 

 ence Internationale pour la Determination des Unites Electriques, seance de la 

 Troisieme Commission, 3 Avril, 1884. There is, however, one essential difference 

 between the present results and those obtained by Violle. In the case both of 

 silver and platinum Violle indicates the " flash " as occurring not before but after 

 the constant period. 



