AND THE ABSORPTION SPECTRUM OF GOLD. 47 



sheets of which the leaves of gold are extended. But this idea is not borne out by 

 microscopic study of that material. The thin gold foil with which the process is 

 begun is first beaten for about twenty minutes only between surfaces of " cutch " 

 paper, which has simply the structure of a felted mass of vegetable fibres. The 

 principal extension of the gold is brought about by beating for about four hours in a 

 " shoder" or packet of leaves of old or previously often used gold-beaters' skin, the 

 packet, containing a thousand leaves, being from time to time bent between the 

 fingers to loosen the gold films and prevent their sticking to the membrane, and 

 finally by beating for another four hours in a " mould" or similarly made up packet 

 of leaves of new or much less used gold-beaters' skin, repeating the bending of the 

 packet to maintain the looseness of the gold films. The cutch is beaten with 

 hammers of about sixteen pounds in weight, striking about sixty blows a minute, the 

 shoder with hammers of about ten pounds and at the rate of about seventy-five blows 

 per minute, and the mould with six-pound hammers and at the rate of about ninety 

 blows per minute. Figs. 9, 10, and 1 1 represent respectively the cutch paper, the 

 already much used gold-beaters' skin of the shoder, and the new, or nearly new, skin 

 of the mould. There is nothing in any of these to account for the black lines seen in 

 the gold-leaf. As far as any distant resemblance to these is suggested by some of 

 the vegetable fibres in fig. ( J, it is to be remembered that fibres in relief would 

 produce in the gold corresponding furrows, appearing as lines of greater, not less, 

 translucency than that of the rest of the surface. The animal membrane or gold- 

 beaters' skin in which by far the greater part of the beating is done, including all the 

 later part of the work, exhibits in figs. 10 and 11 the simple and nearly uniform 

 structure of the serous coat of the intestine said to be the c;t-cum of the ox which 

 is used for the purpose. 



A careful personal inspection of the process of gold-beating at the establishment of 

 the W. H. Kemp Company in New York, has led me to the belief that the production 

 of the ramified lines of microscopic wires or threads in the gold-leaf is due to the 

 following cause. The face of the hammer used is slightly convex, and hence a blow 

 struck with it tends to stretch each sheet of gold, and the animal membrane enclosing 

 it, outwards in all directions from the centre of impact. The membrane is elastic and 

 not absolutely uniform in thickness or tensile strength. Hence it tends to form, 

 along lines of weakness, wrinkles running irregularly outwards, such as may be 

 produced in any stretched piece of cloth by a push of the finger in any given 

 direction. These wrinkles constitute microscopic troughs or furrows into which 

 the soft gold is driven, forming corresponding rods or wires of extremely minute 

 size. The elasticity of the membrane leads to the momentarily developed wrinkles 

 being almost instantly obliterated, while the plasticity of the gold admits of no 

 corresponding disappearance of the wire-like threads produced. The complicated 

 ramification of the lines is no doubt due in part to the irregular distribution of lines 

 of weakness, and therefore of easy stretching, in the membrane, partly to the blows 



