.1889.] On Films produced by Vaporised Metals. 



89 



the late Dr. W. A. Miller, F.R.S., for spectroscopic purposes, and it is 

 remarkably pure. The photographed spectra of this metal yield no 

 lines traceable to gold or any other foreign element. (For its spectrum 

 see 'Phil. Trans.,' 1884, pp. 109 and 134.) Nevertheless I considered 

 that the rose-red and violet tints on the mica films were due to minute 

 traces of gold. Since then, in order to prove or disprove this point, 

 attempts have been made to prepare specimens of perfectly pure silver 

 by the processes of M. Stas, but so far no samples have been obtained 

 which can be relied upon as absolutely free from gold. In one case 

 the chloride precipitated from a cold solution was digested several 

 times with hot aqua regia, washed completely free from soluble 

 matter until the washings had no acid reaction, and subsequently 

 reduced to metal by boiling with pure caustic alkali and milk sugar. 

 This specimen was found to give a characteristic yellow film, but there 

 was a trace of violet-grey surrounding the yellow of the silver, which 

 resembled the colour obtained from metal to which traces of gold 

 had been added, and which became deeper and deeper in colour as 

 the quantity of gold was increased. An alloy was made by melting 

 this sample of silver and adding thereto 10 : \ )0 i b of its weight of pure 

 gold. The alloy was heated to boiling point for a minute ; in order 

 to agitate and completely mix the metals, the fused metal was kept 

 in rapid rotation, and it was granulated by pouring it into water. 

 The cooled metal was next treated in a manner intended to render it 

 homogeneous in composition ; thus, it was hammered out flat, broken 

 in pieces, and again melted, the fused metal being granulated, and the 

 solidified drops being again hammered into disks. By a precisely 

 similar treatment alloys were made from this metal containing pro- 

 portions of gold amounting to no more than xolJo o*h an d tooV oo^ 

 respectively. 



The hammered beads, generally about a decigramme in weight, were 

 submitted to the action of the spark, and deposits of the metal on 

 mica were obtained by passing the spark for 5, 10,' 15, 20, 30, and 

 60 seconds. 



The difference between the deposits from the alloy containing 

 ToVo^h of gold was clearly seen with the naked eye in all cases. The 

 alloy containing Toljo"oth was as clearly seen to contain less gold than 

 the foregoing, and more than that with xo^Vo o* n '■> an( ^ ^ Da * with 

 Y oo^op th gave unmistakeable evidence of gold when examined with a 

 2-inch power under the microscope. The gold tints of rose-red and 

 blue were observed as rings even in cases where the spark had passed 

 for no longer than 5 seconds. Indeed these tests with the shorter 

 duration of the spark gave the best evidence, since too much of the 

 silver deposit obscured the characteristic colours. On the other hand, 

 a much longer exposure of 10 or 20 minutes widely diffused the 

 deposit of gold, and. rendered it evident outside the silver. It may 



