Compoftikn, Purity', and Thidiufs of Gold and Silvn- Lenf, is't-. 133 



inilead of being employed to raife the temperature : and whenever, by continuing the pro- 

 cefs, the tallow already melted began to grow hot, together with the tool, it was eafy to 

 reduce the temperature again by employing the heat on another zone of confident tallow. 

 I ufcd thefe two cylinders with much fatisfaftion, in a confiderable quantity of work. 



In this ftage of experiment, I concluded that the cylinder of pottery mentioned to be ufed 

 in Germany was either of no particular utility beyond that of a common grindftone, or that 

 the report might be inaccurate in this refpefl. But it happened that the fmall (lone here 

 mentioned was laid afide for about three years. At the end of this term it was again 

 brought into ufe, and it was found tliat the tallow had undergone fome change, either 

 from the ftone itfelf, or the a£lion of the external air, which enabled it to defend the grit 

 much more efFeftually than at firft. It feemed to have become lefs fufible. I think it 

 probable that this might not have happened with a pottery cylinder, or at lead that it 

 would have been more eafy to clean and reflore the furface to its firfl ftate. 



2. Concerning Gold, Silver, and other Mdah reduced into very thin Leaves by the Hammer. 



IT is generally thought by chemifts, and others, that leaf gold confifts of the metal in 

 a high ftate of purity. It is never pure, becaufe pure gold is too duclilc to be worked 

 between the gold-beaters' Ikin. The neweft fkins will work the fineft gold, and make the 

 thinneft leaf, becaufe they are the fmootheft. Old fkins, being rough or foul, require 

 coarfer gold. The finer the gold, the more duftile -, infomuch that pure gold, when driven 

 out by the hammer, is too foft to force itfelf over the irregularities, but would pafs round 

 them, and by that means become divided into narrow flips. The fineft gold for this pur- 

 pofe has three grains of alloy in the ounce, and the coarfeft twelve grains. In general 

 the alloy is fix grains, or one-eightieth part. That which is called pale gold contains three 

 pennyweights of filver in the ounce. The alloy of leaf gold is filver, or copper, or both, 

 and the colour is produced of various tints accordingly. Two ounces and two penny- 

 weights of gold is delivered by the matter to the workman, who, if extraordinarily flcilful, 

 returns two thoufand le.ives, or eighty books, of gold, together with one ounce and Jix 

 pennyweights of wafte cuttings. Hence one book weighs 4.8 grains; and as theleav.es 

 rneafure 3.3 inches in the fide, the thicknefs of the leaf is one two hundred and 

 eighty-two thoufandth part of an inch. 



Silver leaf is faid to be pure filver. It is extenfible in this way when compared with gold, 

 rather more than in the proportion of the fpecific gravities. Some leaf filver which I 

 tried was thicker than the gold in the proportion of feven to four. So that the weight of 

 metal covering equal furfac« approached to equality. 



The yellow metal called Dutch gold is fine brafs. It is faid to be made from copper- 

 plates, by cementation with calamine, without fubfequent fufion. Its thicknefs, com- 

 pared with that of leaf gold, proved as 19 to 4, and under equal furfaces it is confiderably 

 more than twice as heavy as the gold. 



The Dutch filver appears to be tin, not only from its habitudes with re-agents, but 

 from the confideration that there is no other cheap white metal of fufficient duftility.' 

 It is fomcwhat more than ten times as thick as gold leaf, and about two and a half times 

 as heavy under equal furfaces. 



3 - The 



