78 THE POPULAR SCIENCE MONTHLY. 



ance sensitive to the twentieth of a milligramme, or the yoVr of a 



grain. 



No. 1 weighed 126-j^fu% grains troy. 



A gold "blush" of sufficient thickness to produce a fine gold- 

 color was then deposited by the battery. The plates were washed 

 in distilled Avater, dried, and reweighed without rubbing, and were 

 found to have each gained in weight exactly one-tenth (yL) of a grain. 

 It thus appears that one grain of gold may be distributed, by the 

 galvanic deposit, over the surface of two hundred square inches, as 

 contrasted with seventy-five square inches by beating. In other 

 words, the metal is more than two and a half times thinner in the 

 former case than in the latter, or 330^400 P^^^'*^ ^^ ^" ^"^'^^> ^^ compared 

 with -g- --f'eTo" ^^ ^'^ inch. 



A still thinner deposit of gold could, of course, be detected on 

 the delicate assay-balance, but, owing to the transparency of the 

 film, it would not possess the true gold-color. It seemed important 

 to ascertain whether the gold was evenly distributed over the cop- 

 per surface, or whether it was deposited in spots. The strips were 

 accordingly examined under a microscope. 



A careful examination showed that there were no exposed surfaces 

 of copper, and the gold appeared to have a fine, bright, smooth sur- 

 face. This, however, was not considered sufficient proof, and several 

 expedients were tried to obtain the gold films free from the copper 

 plate, in order tliat they might be examined by transmitted light. 

 Owing to their extreme thinness this was difficult to accomplish. One 

 method, which was partially successful, was to heat the copper plates 

 to a cherry red in the annealing muffle of an assay-furnace. On cool- 

 ing, the gold film peeled off in flakes with a thin backing of oxide of 

 copper ; these flakes were pressed between two plates of glass, and 

 nitric acid allowed to flow in by capillary action. The acid dissolved 

 the copper; leaving a film of free gold, Tlie difficulty was, tliat the 

 bubbles of gas formed perforated the film of gold. Another plan 

 was then tried. The gold-plating was removed from one surface of 

 the copper plates by means of fine emory-paper. Pieces about one 

 inch square were immersed for several days in very weak nitric acid. 

 The copper was entirely dissolved, and detached films of gold were 

 found floating intact on the surface of the liquid ; these were carefully 

 collected on strips of glass, washed with distilled water, and dried ; 

 they then firmly adhered to the glass. 



When examined by reflected light they retain their brilliant gold 

 color and lustre, but when viewed by transmitted light they are 

 bright green and very transparent ; the color is an even shade, having 

 none of the mottled appearance of gold-leaf when seen by transmit- 

 ted light, caused by its very uneven thickness. This monotone ap- 

 pears to be a positive indication of uniform thickness, for, when two 



