98 FRAGMENTS OF SCIENCE 



cal precipitates of various kinds, and found that they one 

 ^nd 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, 1 

 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 

 ^%j 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 



