272 ANNUAL OF SCIENTIFIC DISCOVERY. 



covered with a layer about one-tenth of an inch in depth, which easily stands 

 upon it, and in that state its temperature is allowed to rise. In about ten 

 minutes, by the heat of warm water in the hollow box of the table, silver 

 begins to deposit on the glass, and in fifteen or twenty minutes a uniform 

 opaque coat, having a greyish tint on the upper surface, is deposited. After 

 a certain time, the glass employed in the illustration was pushed to the edge 

 of the table, was tilted that the fluid might be poured off, then washed with 

 water, and examined. The under surface presented a .perfectly brilliant 

 metallic plate of high reflective power, as high as any that silver can attain 

 to ; and the coat of silver, though thin, was so strong as to sustain handling, 

 and so firm as to bear polishing on the back to any degree, by rubbing with 

 the hand and polishing powder. 



" The usual course in practice, however, is when the first stratum of fluid is 

 exhausted, to remove it, and apply a layer of No. 2 solution, and when that 

 has been removed and the glass washed and dried, to cover the back surface 

 with a protective coat of black varnish. When the form of the glass varies, 

 simple expedients are employed, and either concave, convex, or corrugated 

 surfaces are silvered, and bottles and vases are coated internally. It is easy 

 to mend an injury in the silvering of a plate, and two or three cases of repair 

 were performed on the table." 



SOME OBSERVATIONS ON DIVIDED GOLD. 



The following is an abstract of a paper recently read before the Royal Insti- 

 tution of Great Britain, by Prof. Faraday : 



The author has been led by various considerations to seek experimentally for 

 some effect on the rays of light by bodies which when in small quantities had 

 strong peculiar action upon it, and which also could be divided into plates and 

 particles so thin and minute as to come far within the dimensions of an undula- 

 tion of light, whilst they still retained more or less of the power they had in mass. 

 The vibrations of light are for the violet ray 59,570 in an inch, and for the red ray 

 37,640 in an inch ; it is the lateral portion of the vibration of the ether* which 

 is by hypothesis supposed to affect the eye, but the relation of number remains 

 the same. Now a leaf of gold as supplied by the mechanician is only 2 -^~ To 

 of an inch thick, so that 1\ of these leaves might be placed in the space occu- 

 pied by a single undulation of the red ray, and 5 in the space occupied by a 

 violet undulation. Gold of this thickness and in this state is transparent, 

 transmitting green light, whilst yellow light is reflected; there is every reason 

 to believe also that some is absorbed, as happens with all ordinary bodies. 

 When gold leaf is laid upon a layer of water on glass, the water may easily 

 be removed and solutions substituted for it ; in this way a solution of chlorine, 

 or of cyanide of potassium, maybe applied to thin the film of gold; and as the 

 latter dissolves the other metals present in the gold (silver for instance, which 

 chlorine leaves as a chloride), it gives a pure result ; and by washing away the 

 cyanide, and drying and draining the last remains of water, the film is left 



* Analogous transverse vibration may easily be obtained on the surface of water or 

 other fluids by the process described in the Philosophical Transactions for 1831, p. 

 386, &c. 



