

GOLD-BEATERS' SKIN 727 



to water ; heats the crucible to dull redness in the muffle, when the gold assumes the 

 metallic lustre and the cornet becomes solid. It is now taken out of the crucible and 

 weighed. 



When the alloy contains platinum, the assay presents greater difficulties. In general, 

 to separate with accuracy the platinum from the gold, we must avail ourselves of a 

 peculiar property of platinum ; when alloyed with silver, it becomes soluble in nitric 

 acid. Therefore, by a proper quartation of the alloy by cupellation, and boiling the 

 button with nitric acid, we may get a residuum of pure gold. If we were to treat the 

 button with sulphuric acid, however, we should dissolve nothing but the silver. The 

 copper is easily removed by cupellation. 



The examination may also be employed for assay purposes to indicate the quantity 

 of silver necessary for quartation or parting. 



b. Silver Parting. This method is employed for estimating gold in certain varieties 

 of silver. When an alloy of silver and gold contains about 65 per cent, or more of silver, 

 the silver may be completely removed by the action of nitric acid. 20 grs. of the 

 silver are to be weighed out, transferred direct to the parting flask, and boiled in the 

 first and second nitric acids for about the same time and in the same manner as for 

 gold parting. The residual gold, which is generally left in the state of fine powder, is 

 very carefully washed with water., transferred to a crucible, dried, ignited and weighed. 

 If the silver under examination contains lead, antimony, and other impurities, it is 

 first cupelled with lead and the button afterwards cleaned, and flattened out, before 

 submitting it to the parting process. 



c. Assay by the Touchstone or the Touch. This method is used by jewellers and 

 others to ascertain the approximate fineness or quality of gold trinkets, &c., and to 

 prevent fraud. It depends on comparing the colour and other properties of a minute 

 portion of the metal tested, with those of small bars of alloys, the composition of which 

 is known. For this purpose we require a Touchstone of a black colour, consisting of 

 Lydian stone, pottery, or other substance ; and a series of needles or alloys of gold and 

 copper, gold and silver, or gold silver and copper, &c. Thus the set of gold and copper 

 needles may consist of pure gold ; 23 carats of gold with \ carat of silver ; 23 carats 

 of gold with 1 carat of silver ; 22 carats of gold with \\ carat of silver; and so on, 

 till the silver amounts to four carats ; after which the addition may proceed by whole 

 carats. A Test acid, consisting of nitric acid, or of nitric acid with about 2 per cent, 

 of hydrochloric acid, is used. The examination may be made as follows : First remove 

 the superficial coating of gold, by rubbing or otherwise. Then compare the streak pro- 

 duced by rubbing the article upon the touchstone with those produced from the alloys. 

 When the one is found which most closely corresponds in colour, both streaks are 

 tested with the acid, and a judgment formed therefrom. 



The attentive assayer compares not only the colour of the stroke made upon the 

 touchstone by the metal under examination with that produced by the needle, but will 

 likewise attend to the sensation of roughness, dryness, smoothness, or greasiness, which 

 the texture of the rubbed metal excites when abraded by the stone. When two strokes 

 perfectly alike in colour are made upon the stone, the test acid will affect them very 

 differently if they are not of similar compositions. The stone itself may also be made 

 red-hot by the fire, or by the blowpipe, if thin black pottery be used ; in which case 

 the phenomena of oxidation will differ according to the nature and quantity of the alloy. 



GOLD, ALLOYS OF. See AIXOY. 



GOLD-.aiVTALG.aiVI. See AMALGAM. 



GOLB-SEATERS' SKIN*. This skin is prepared from the external or peritoneal 

 coat of the caecum or blind gut of neat cattle. The workman separates and turns 

 over the portion which encircles the junction of this pouch with the rest of the intes- 

 tines, and draws it off inverted from the other coats to the length of 25 or 30 inches. 

 It is then immersed a short time in a weak solution of potash, and is cleaned by 

 straying upon a board with a knife. When thus well cleaned, and by soaking in 

 water, the piece is stretched upon a kind of frame from 40 to 50 inches in length and 

 1 1 inches wide, and made up of two uprights held together by two cross-pieces having 

 longitudinal grooves two and a half lines in width. The surface of the membrane, 

 which was outside in the animal, is placed in contact with the upper part of the 

 frame ; it is stretched in every direction, and is glued to its rim. Another membrane 

 is then stretched above the first, with its external surface placed upwards, and is at- 

 tached to it by glueing around the edges. When dry, the membranes are separated by 

 running a sharp knife along the grooves. Each strip is then glued upon a frame 

 similar to the first one, but without a groove, and is washed over with a solution com- 

 posed of alum, 1 ounce ; water, 2 quarts. 



When the surface is dried, a sponge dipped in a concentrated solution of fish-glue 

 in white wine, rendered aromatic by cloves, nutmegs, or camphor, is passed over it. 

 When this coating is dried, it is covered with a coat of white of eggs, and the strip is 



