398 On the Experimental Relations [1857. 



It may be thought that the beating has conferred a uniform 

 strained condition upon the gold, a difference in quality in one 

 direction which annealing takes away ; but when the gold is 

 examined by polarized light, there is no evidence as yet of such 

 a condition, for the green and the colourless gold present like 

 results ; and there is a little difficulty in admitting that such 

 an irregular corrugated film as gold-leaf appears to be can pos- 

 sess any general compression in one direction only, especially 

 when it is considered that it is beaten amongst tissues softer 

 than itself, and made up with it into considerable masses. The 

 greening effect of pressure occurs with the deposited particles 

 of electric discharges, and here it appears either amongst the 

 larger particles near the line of the discharge, or amongst the 

 far finer ones at a considerable distance. Such results do not 

 suggest a dependence upon either the size of the particles or 

 their quantity, but rather upon the relative dimensions of the 

 particles in the direction of the ray and transverse to that di- 

 rection. One may imagine that spherical or other particles, 

 which, being disposed in a plane, transmit ruby rays or violet 

 rays, acquire the power, when they are flattened, of transmitting 

 green rays, and such a thought sends the mind at once from 

 the wave of light to the direction and extent of the vibrations of 

 the ether. For it does not seem likely that pressure can pro- 

 duce its peculiar result by affecting the relation of the dimension 

 of the particle to the length-dimension of a progressive undu- 

 lation of light, the latter being so very much greater than the 

 former ; but the relation to the dimension of the direct or lateral 

 vibration of the particles of the ether may be greatly affected, 

 that being probably very small and much nearer to, if not even 

 less than, the size of the particles of gold. 



Silver-leaf, as usually obtained by beating, is so opake, as 

 perfectly to exclude the light of the sun. When this is laid by 

 water on plates of rock-crystal and heated in a muffle, it begins 

 to change at a temperature lower than that required for gold, 

 and becomes very translucent, losing at the same time its re- 

 flective power : it looks very like the film of chloride produced 

 when a leaf of silver is placed in chlorine gas. When examined 

 by a lens or an ordinary microscope, the leaf seems to be as 

 continuous as in its original state ; the finest hole, or the finest 

 line drawn by a needle point, appears only to prove the con- 



