394 On the Experimental Relations [1857. 



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 o.i thrown back. Latterly I 

 have obtained some pure gold-leaf beaten by Marshall, of which 

 5000 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 5 per cent, of silver pro- 

 duces 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 ^th part of a single wave of light. By 

 chemical 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 edge of the leaf, it will pass between the glass 

 and gold, and the latter will be perfectly stretched ; if the water 

 be then drained out, the gold-leaf will be left well extended, 

 smooth, and adhering to the glass. If, after the water is poured 

 off, a weak solution of cyanide be introduced beneath the gold, 

 the latter will gradually become thinner andjthinner ; but at 

 any moment the process may be stopped, the cyanide washed 

 away by water, and the attenuated gold film left on the glass. 

 If towards the end a washing be made with alcohol, and then 

 with alcohol containing a little varnish, the gold film will be 

 left cemented to the glass f. 



* The chlorine leaves a film of chloride of silver behind, the cyanide leaves 

 only metal. 



f Air-voltaic circles are formed in these cases, and the gold is dissolved 

 almost exclusively under their influence. When one piece of gold-leaf was 

 placed on the surface of a solution of cyanide of potassium, and another, 

 moistened on both sides, was placed under the surface, both dissolved ; but 

 twelve minutes sufficed for the solution of the first, whilst above twelve hours 

 were required for the submerged piece. In weaker solutions, and with silver 

 also, the same results were obtained ; from sixty to a hundredfold as much 

 time being required for the disappearance of the submerged metal as for that 

 Which, floating, was in contact both with the air and the solvent. An action 



