Heat radiated by Rough and by Bright Surfaces. 



85 



When, however, a plate of alum was interposed, the propor- 

 tion of rays transmitted from the two sources became quite sur- 

 prisingly different — so much so, indeed, that the absolute quantity 

 of heat transmitted from the platinized plate scarcely exceeded 

 the quantity transmitted from the bright plate. 



It is scarcely needful to mention that the experiments were 



extended to more than one plate of alum. This appeared so much 



the more necessary, since the total quantity of heat which passes 



through alum amounts only to a few per cent. The same result 



was always obtained with six perfectly different plates, varying 



in thickness from 1*5 millim. to 9*5 millims. With all of them 



z z 



j was greater than ~' } and although the quantities of heat which 



were allowed to pass varied greatly, according to the thickness 

 and condition of the plates, the heat transmitted by each plate 

 from the two sources was nearly the same. 



The quantities of heat transmitted, for every 100 parts which 

 fell upon the plates, were as follows : — 







From the 



From the 



Number of 





bright plati- 



platinized 



the plate. 



Thickness. 



num, 

 i 



platinum, 

 h 







T 



h 





millims. 







I 



1-5 



7-77 



4-34 



2 





416 



2-33 



3 



6-5 



407 



2'32 



4 



80 



407 



2-28 



5 



9-5 



300 



173 



6* 



9-25 



314 



1-9 



Plates of copal of various kinds and of various thicknesses 



behaved in the same way as plates of alum. With them also j 



differed very considerably from j-, but the absolute quantities of 



heat which passed through were not so nearly equal as in the 

 case of alum. 



Since the proportional quantities of the heat from the two 

 disks which is absorbed by the several plates of alum and of 

 copal, is different, it follows either that the platinized disk must 

 emit heat of a different colour from that radiated by the other 

 disk, or that the increase in the intensity of the radiation caused 

 by the platinizing does not affect all colours in the same propor- 

 tion, or, again, that both these effects are produced at once, 



* Rather less clear than No. 5. 



