98 FRAGMENTS OF SCIENCE 



cal precipitates of various kinds, and found that they one 

 and all manifested the same power of radiation. They 

 concluded from their researches, that when bodies are re- 

 duced to an extremely fine state of division, the influence 

 of this state is so powerful as entirely to mask and over- 

 ride whatever influence may be due to chemical consti- 

 tution. 



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 lustre behind; and this varnish, though so per- 

 fectly transparent to luminous rays, is as opaque as pitch, 

 or lamp-black, to non-luminous ones. It is a powerful 

 emitter of dark rays; it is also a powerful absorber. 

 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, experi- 

 mented thus: they mixed their powders and precipitates 

 with gum- water, and laid them, by means of a brush, upon 

 the surfaces of a cube like this. True, they saw their red 

 powders red, their white ones white, and their black ones 



