68 FRA GMENTS OF SCIENCE. 



to an extremely fine state of division^ the influence of this 

 state is so powerful as entirely to mask and override what- 

 ever influence may be due to chemical constitution. 



But it appears to me that through the whole of these 

 researches an oversight has run, the mere mention of 

 which will show what caution is essential in the opera- 

 tions of experimental philosophy; while an experiment or 

 two will make clear wherein the oversight consists. Fill- 

 ing a brightly polished metal cube with boiling water, I 

 determine the quantity of heat emitted by two of the 

 bright surfaces. As a radiator of heat one of them far 

 transcends the other. Both surfaces appear to be metallic; 

 what, then, is the cause of the observed difference in their 

 radiative power? Simply this: one of the surfaces is 

 coated with transparent gum, through which, of course, is 

 seen the metallic luster behind; and this varnish, though 

 so perfectly transparent to luminous rays, is as opaque as 

 pitch, or lampblack, to non-luminous ones. It is a 

 powerful emitter of dark rays; it is also a powerful ab- 

 sorber. While, therefore, at the present moment, it is 

 copiously pouring forth radiant heat itself, it does not 

 allow a single ray from the metal behind to pass through 

 it. The varnish then, and not the metal, is the real 

 radiator. 



Now Melloni, and Masson, and Courtepee experimented 

 thus: they mixed their powders and precipitates with gum- 

 water, and laid them, by means of a brush, upon the sur- 

 faces of a cube like this. True, they saw their red pow- 

 ders red, their white ones white, and their black ones 

 black, but they saw these colors through the coat of var- 

 nish which surrounded every particle. When, therefore, it 

 was concluded that color had no influence on radiation, no 

 chance had been given to it of asserting its influence; 

 when it was found that all chemical precipitates radiated 

 alike, it was the radiation from a varnish, common to 

 them all, which showed the observed constancy. Hun- 

 dreds, perhaps thousands, of experiments on radiant heat 

 have been performed in this way, by various inquirers, 

 but the work will, I fear, have to be done over again. I 

 am not, indeed, acquainted with an instance in which an 

 oversight of so trivial a character has been committed by 

 so many able men in succession, vitiating so large an 

 amount of otherwise excellent work. 



