1857.] of Gold (and other Metals) to Light. 393 



especially fitted for experiments of this nature, because of its 

 comparative opacity amongst bodies, and yet possession of a 

 real transparency ; because of its development of colour both 

 in the reflected and transmitted ray; because of the state of 

 tenuity and division which it permitted with the preservation 

 of its integrity as a metallic body ; because of its supposed 

 simplicity of character ; and becauseknown phenomena appeared 

 to indicate that a mere variation in the size of its particles gave 

 rise to a variety of resultant colours. Besides, the waves of 

 light are so large compared to the dimensions of the particles 

 of gold which in various conditions can be subjected to a ray, 

 that it seemed probable the particles might come into effective 

 relations to the much smaller vibrations of the ether particles ; 

 in which case, if reflexion, refraction, absorption, &c., depended 

 upon such relations, there was reason to expect that these 

 functions would change sensibly by the substitution of different 

 sized particles of this metal for each other. At one time I hoped 

 that I had altered one coloured ray into another by means of 

 gold, which would have been equivalent to a change in the num- 

 ber of undulations ; and though I have not confirmed that result 

 as yet, still those I have obtained seem to me to present a useful 

 experimental entrance into certain physical investigations re- 

 specting the nature and action of a ray of light. I do not pretend 

 that they are of great value in their present state, but they are 

 very suggestive, and they may save much trouble to any experi- 

 mentalist inclined to pursue and extend this line of investigation. 



Gold-leaf effect of heat, pressure, $c. 



Beaten gold-leaf is known in films estimated at the 28 / uo0 th 

 of an inch in thickness ; they are translucent, transmitting green 

 light, reflecting yellow, and absorbing a portion. These leaves 

 consist of an alloy in the proportions of 12 silver and 6 copper 

 to 462 of pure gold. 2000 leaves 3ths of an inch square are 

 estimated to weigh 384 grains. Such gold-leaf is no doubt full 

 of holes, but having, in conjunction with Mr. W. De la Rue, 

 examined it in the microscope with very high powers (up to 

 700 linear), we are satisfied that it is truly transparent where 

 the gold is continuous, and that the light transmitted is green. 

 By the use of the balance Mr. De la Rue found that the leaf 

 employed was on the average irwDirth of an inch thick. 



