1874.] Iby [Outerbridge. 



in a spectroscopic assay, (should that ever be possible) the following ex- 

 periment was tried. Having weighed small electrodes, averaging 18 

 milligrammes each, with the greatest possible accuracy on the gold assay 

 balance of the Mint, (which is sensitive to a twentieth of a milligramme, 

 or even less,) and having arranged a spark register, I found that 1000 

 sparks might be passed between these poles, each spark showing the 

 spectrum of the metal distinctly, and yet the loss in weight was too small 

 to be made the base of calculation. Thus, a gold pole lost in weight after 

 passing 1000 sparks, -j-j^g^ of a grain ; this gives for each spark xj, o o o o o 

 of a grain of gold, producing a bright spectrum. I increased the number 

 to 3000 sparks as the test. The loss of weight depends of course upon 

 the electric volume, and in the experiments tabulated I endeavored to 

 keep the latter constant. A slight deposit of the vaporized metal from 

 the opposite pole takes place in fine division, but this is easily re- 

 moved — in the case of copper and gold poles by dipping the gold for a 

 moment in weak acid, or by gentle rubbing. The annexed tables (marked 

 A and B) show that the loss in weight is marvellously small, averaging 

 less than seven-tenths of a milligramme of gold for 3000 sparks. To give 

 the amount for each spark, this must be divided by the number of sparks ; 

 thus, in round numbers an electrode loses xcfo o of a grain after passing 

 3000 sparks ; or for 1000 sparks joV o of a grain, or for each spark yo^^oZ) o 

 of a grain. The exceedingly small quantity of metal thus assayed ren- 

 ders this process, to my mind, inapplicable to the operations in the Mint ; 

 for it is necessary to determine gold assays to the roJo^ P^^* of the nor- 

 mal assay weight, and it is hardly conceivable that a discrimination to 

 the 1^0 0^ part of the spark assay weight, or the tooT'oo ooooo of a grain is 

 practically possible. Even if it were, it would not be proper to assume 

 that a test on such an atomic scale would correctly represent the value 

 of a large deposit, or even of gold ingots. It would certainly not be in 

 the case of silver, which segregates. 



The table of loss shows another curious and unexpected result, viz. : 

 that the loss in weight of the volatile metals very slightly exceeds and in 

 some cases does not equal the loss of the less volatile metals. Thus, in 

 three different experiments of 3000 sparks each, copper loses but .1 M. while 

 gold loses .5 M. It must be remembered that in these experiments a much 

 stronger spark was used than was necessary to show a visible spectrum. 

 When reduced to a minimum, as was done in the case of the miniature 

 Leyden jar, which still gave a distinct spectrum, the loss in weight after 

 3000 sparks, for silver, copper and tin, was absolutely inappreciable on 

 the balance. 



An unexplained anomaly was also noticed in relation to the sensitive- 

 ness of the spectroscope to the metals present in small quantity. Although 

 Mr. Cappel has shown, by passing the spark through weak solutions of 

 pure metals, that ^o^oo of a milligramme of gold will show a spectrum, 

 (it is even less than ^^-f^g^ of a M. according to an experiment perfoi-med 

 by the method described above) yet a comparatively large proportion of 



A. P. S, — VOL. XIV. V 



