146 
DE. FAEADAT ON THE EXPEEDIENTAL EELATIONS 
458 to 727 billions; but the extent of the lateral excursion of the particles of the ether, 
either separately or conjointly, is not known, though both it and the velocity are pro- 
bably very small compared to the extent of the wave and the velocity of its propagation. 
Colour is identified with the number of waves. '\Miether refiexion, refi’action, &c. have 
any relation to the extent of the lateral vibration, or whether they are dependent in part 
upon some physical action of the medium unknoum to and unsuspected hy us, are points 
which I understand to be as yet undetermined. 
Conceiving it very possible that some experimental evidence of value might result from 
the introduction into a ray of separate particles having great power of action on light, 
the particles being at the same time very small as compared to the wave-lengths, I sought 
amongst the metals for such. Gold seemed 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 refiected and transmitted 
ray ; because of the state of tenuity and division which it permitted uith the preserva- 
tion of its integrity as a metallic body ; because of its supposed simphcity of character ; 
and because known phenomena appeared to indicate that a mere variation hi the size 
of its particles gave rise to a variety of resultant colours. Besides, the waves of hght 
are so large compared to the dimensions of the particles of gold which in various con- 
ditions can be subjected to a ray, that it seemed probable the particles might come into 
efiective 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 number of undulations ; and though I have not confiimied that result as 
yet, still those I have obtained seem to me to present a useful experimental entrance 
into certain physical investigations respecting the nature and action of a ray of hght. 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 experinientahsts inchned to pimsue 
and extend this line of investigation. 
Gold-leaf — effect of heat, pressure, ffr. 
Beaten gold-leaf is known in films estimated at the 2^0 of an inch in thickness ; 
they are translucent, transmitting green light, reflecting yellow, and absorbmg a portion. 
These leaves consist of an alloy in the proportions of 12 silver and 6 copper to 462 
of pure gold. 2000 leaves 3|ths of an inch square are estimated to weigh 384 grains. 
Such gold-leaf is no doubt full of holes, but having, in conjunction Avith ]Mr. ^1'. De la 
Hue, examined it m the microscope Avith A*ery high poAA'ers (up to 700 linear), Ave are 
satisfied that it is truly transparent Avhere the gold is continuous, and that the light 
transmitted is green. By the use of the balance Mr. De la Hue found that the leaf 
employed was on the average of an inch thick. Employing polarized light 
