PRODUCED BY ELECTRO-CHEMICAL ACTION. 107 



which depend on the tenuity of plates are not to be traced on all classes 

 of bodies ; that they can be produced by those bodies only which are 

 endowed with a certain degree of transparency ; and that metallic sub- 

 stances are too opaque to be numbered among these. This is a positive 

 fact, and ought therefore, without any regard to particular systems, to 

 be entered in the register of science. 



Gold and Copper. 



It cannot be doubted, says Newton, that the colours of gold and 

 copper belong to the second or the third order *. To us they seem, on 

 the contrary, to belong to the first order, that being the only one which 

 includes tints of a metallic appearance. If we only recollect that the 

 first colours of the scale are far from being distinct in the first of New- 

 ton's rings, we shall feel less surprised that it should be necessary to 

 correct the classification of that great philosopher. The resemblance 

 in question is, however, as we have observed already, very far from 

 being perfect. The tints that come nearest to the yellow of gold are 

 the blond colours Nos. 2 and 3 : but these are evidently less yellow, 

 and at the same time more compounded than the colour of gold ; for 

 they contain a tinge of green, which does not exist in the more decided 

 colour of gold. Transparent gold-leaf appears green when held before 

 the light : this fact has been classed by several persons among the 

 phsenomena connected with thin laminae, because these laminae are 

 known to reflect a given colour, in the same position in which they 

 transmit its complementary colour. However I will say with a great 

 philosopher, that " there is in Newton's rings no yellow that has green 

 for its complement : the colour transmitted is invariably the blue ; and 

 this fact accords with the construction given by Newton for the com- 

 position of colours. But extract from this blue (which is necessarily 

 compounded) a certain number of violet and blue rays, such as may be 

 absorbed by the substance of gold, and there will remain greenf ." 



It is a fact demonstrated by a great number of observations, that 

 light in its passage through coloured substances is partially absorbed 

 and extinguished. This fact not only renders Biot's explanation plausi- 

 ble, but warrants the supposition that light undergoes in reflexion a 

 diminution analogous to that which takes place in its transmission. 

 For if some of the rays destined to be transmitted are absorbed by the 

 very substance of the gold, how can all the other rays, which are de- 

 stined to be reflected in the interior of the same substance, escape undi- 

 minished ? If the phaenomenon be incomplete in respect to transmission, 

 it will be equaUy so in respect to reflexion, and the tint formed will be 



* Optics, Book II. part 3. prop. 5. 



\ Biot, Traile de Physique, vol. iv. p. 127. 



