of Gold and other Metals to Light. 403 
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 inthe number of undu- 
lations ; and though I have not confirmed that result as yet, still 
those I have obtained seem to me to present a useful experimen- 
tal entrance into certain physical investigations respecting 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 sug- 
gestive, and they may save much trouble to any experimentalists 
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 y,/555th 
of an inch in thickness ; they are translucent, transmitting green 
hght, reflecting yellow, and absorbing a portion. These leaves 
consist of an alloy in the proportions of 12 silver and 6 cop- 
per to 462 of pure gold. 2000 leaves 33ths 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 77,4,55th of an inch thick. 
Employing polarized light and an arrangement of sulphate of 
lime plates, it was found that other rays than the green could be 
transmitted by the gold-leaf. The yellow rays appeared to be 
those which were first stopped or thrown back. Latterly I have 
obtained some pure gold-leaf beaten by Marshall, of which 2000 
leaves weighed 408 grains, or 0-2 of a grain per leaf; its reflected 
colour is orange-yellow, and its transmitted colour a warm green. 
Gold alloy containing 25 per cent. of silver produces pale gold- 
leaf, which transmits a blue purple light, and extinguishes much 
more than the ordinary gold-leaf. 
So a leaf of beaten gold occupies in average thickness no more 
than from jth to jth part of a single wave of light. By chemi- 
cal means, the film may be attenuated to such a degree as to 
transmit a ray so luminous as to approach to white, and that in 
parts which have every appearance of being continuous in the 
microscope, when viewed with a power of 700. For this purpose 
it may be laid upon a solution of chlorine, or, better still, of the 
cyanide of potassium*. If a clean plate of glass be breathed 
upon and then brought carefully upon a leaf of gold, the latter 
will adhere to it ; if distilled water be immediately applied at the 
* The chlorine leaves a film of chloride of silver behind, the cyanide 
leaves only metal. D2 
2D2 
