^e THE POPULAR SCIENCE MONTHLY. 



hundred and fifty leaves, " each leaf being four fingers square," or 

 about three times thicker than our ordinary gold-leaf. On the cof- 

 fins of the Theban mummies, gold-foil has been discovered of extraor- 

 dinary thinness. The ancient Peruvians covered the walls of their 

 temples with very thin sheets of gold, and the rude specimens of gild- 

 ing on the palace of Tippo Saib, at Bungalore, prove that the art of 

 gold-beating was practised in India. We also have Biblical author- 

 ity for the antiquity of the art.' 



Experiments made in modern times have shown that a single grain 

 of gold may be beaten out so as to cover a space of seventy-five 

 square inches ; the thickness of the leaf is then only the three hun- 

 dred and sixty-seven thousand six hundred and fiftieth (-j-btWo) 

 part of an incli, or about twelve hundred times thinner than an ordi- 

 nary sheet of printing-paper. 



Faraday states in his researches on " The Experimental Relations 

 of Gold (and otiier Metals) to Light " ^ that a leaf of beaten gold 

 occupies an average thickness of no more than \ to |- part of a single 

 wave of light. He reduced the thickness of gold-leaves at pleasure, 

 by spreading them upon glass plates and gradually dissolving the 

 metal by means of a weak solution of cyanide of potassium. "By 

 this means," he says, " I think fifty or even one hundred might be 

 included in a single progressive undulation of light." ^ 



Faraday's researches upon the nature of thin films of gold and 

 other metals, and upon the size of finely-divided particles of gold 

 diflfused through various liquids, are of a most interesting and refined 

 character. Availing himself of the well-known reducing power of 

 phosphorus, he floated small particles of it upon the surface of weak 

 solutions of chloride of gold. In the course of twenty-four hours 

 he found that the surfaces of the liquids were covered with films 

 of metallic gold, which were thicker near the pieces of phosphorus 

 " possessing tlie full golden reflective power of the metal," but 

 becoming so thin by gradations as to be scarcely perceptible. " They 

 acted as thin plates upon light, producing the concentric rings of 

 colors round the phosphorus at their first formation, though their 

 thickness then could scarcely be the ydt, perhaps not the ^-i^, of a 

 wave-undulation of light." 



By treating very dilute solutions of gold with phosphorus he ob- 

 tained the metal diff"used through the liquid in extremely fine particles, 

 producing a beautiful ruby-color. These particles, " when in their 

 finest state, often remain unchanged for months, and have all the ap- 

 .pearance of solutions, but they never are such, containing, in fact, no 

 dissolved but only diffused gold. The particles are easily rendered evi- 

 dent by gathering the rays of the sun (or a lamp) into a cone by a lens, 



"And they did boat the j:;old into thin plates" (Exodus xxxix. 3). 

 " Philosophical Transactions," 1857. 

 2 Faraday's " Researches in Chemistry and Physics." 



