414 Prof. Faraday on the Experimental Relations 
film will be left floating. After sufficient washing it may be 
taken up in portions on smaller plates of glass, dried, and kept 
for use. Mr. Warren De la Rue taught me how to make and 
deal with these films: they may by attention be obtained very 
uniform, of very different degrees of thickness, from almost 
perfect transparency to complete opacity, and by successive 
application of the same collecting glass plate may be superposed 
with great facility. 
These films may be examined either on the water or on the 
glass. When thick, their reflective power is as a gold plate, 
full and metallic; as they are thinner they lose reflective power, 
and they may be obtained so thin as to present no metallic 
appearance, all the coloured rays of light then passing freely 
through them. As to the transmitted light, the thinner films 
generally present one kind of colour ; it appears as a feeble gray- 
violet, which increases in character as the film becomes thicker 
and sometimes approaches a violet ; a greenish-violet also appears; 
and the likeness of the gray-violet tint of these films to the stains 
produced by a solution of gold on the skin or other organic 
reducing substance, or the stain produced on common pottery, 
cannot be mistaken. Superposition of several gray-violet films 
does not produce a green tint, but only a diminution of light 
without change of colour. In those specimens made by par- 
ticles of phosphorus floating on the solution of gold, very fine 
green tints occur at the thicker and golden parts of the film. 
The colour of the gold here may depend in some degree on the 
manner in which these films are formed: the thicker parts are not 
produced altogether by the successive addition of reduced gold 
from the portion of fluid immediately beneath them. When a 
particle of phosphorus is placed on pure water, it immediately 
throws out a film which appears to cover the whole of the sur- 
face; in a little while the film thickens around the particle and 
is easily distinguished by its high reflective power. It is this 
film which reduces the gold in solution, being itself consumed in 
the action; the result is a continued extension from the phos- 
phorus outwards, which, after it has covered the solution with a 
thin film of gold, continues to cause a compression of the parts 
around the phosphorus and an accumulation there, rendering 
the gold at a distance of half an inch from the phosphorus so 
thick, that it is brilliant by reflexion and nearly opake by trans- 
mission, whilst near to the phosphorus the forming film is 
so thin as to be observed only on careful examination, and is still 
travelling outwards and compressing the surrounding parts more 
and more. The phosphorus is very slowly consumed; a par- 
ticle not weighing ;3,dth of a grain will remain for four or five 
days on the surface of water before it disappears. 
