498 



Mr. j. E. Petavel. An Experimental 



11. — Superficial area of the platinum ingot, 10 sq. cm. (or 41 per cent, below the normal) 

 intensity of the light at the temperature of solidification, 1'004 (or 0'004 above the normal). 



i-0 



O JO 20 30 40 50 60 70 80 90 100 110 120 ISO 140 150 i6Q 



Time in Seconds. 



the limiting values. A number of determinations were made with a 

 superficial area of 30 and 10 sq. cm. The details of two of the 

 determinations are given in Tables VI and VII ; the form of the 

 curves is shown in figs. 9 and 11. 



The maximum temperature reached by the metal was not sufficient 

 to cause the sharp break in the curve of light, but notwithstanding 

 this fact, the values obtained differ less than 1J per cent, from the 

 " normal " value. We are therefore justified in concluding that the 

 intensity of the light is independent of the shape of the mass, so 

 long as the entire mass is raised above its melting point. 



Variation of the mass of metal used. — The effect of increasing the 

 quantity of platinum from 345 to 510 grammes is shown in Table 

 VIII and fig. 12. In this case the change produced in the light 

 works out at 0*4 per cent. It is not possible to decrease the mass of 

 metal much below 345 grammes, as the " constant period " becomes too ' \ 

 short for reliable readings to be obtained. For 90 grammes, for instance 

 (see fig. 13), the "constant period" has altogether disappeared, or, 

 strictly speaking, it is only represented by a slight inflection in the 

 curve. The experiments are, however, sufficient to show that though 

 the mass of the ingot has a considerable effect on the degree of 

 accuracy obtainable, it does not affect the quantity of light emitted at 

 the temperature of solidification. 



Variation in the shape of the enclosure. — From the laws of thermal 

 radiation we are led to expect that the shape and temperature of the 

 enclosure will have a considerable influence on the quantity of light 

 emitted by such a body as platinum. Any change in size of the 

 aperture in the nover of the furnace will greatly modify the rate of 

 cooling of the metal, and from this cause also might affect the photo- 



