172 A. W. Wright—Electrical Deposition of Metals. 
joined in a continuous circuit. By immersing the plates of the 
plunge battery more or less, as well as by varying the number 
im the circuit, the strength of the current could readily be 
changed within the limits desired. The various metals required 
currents of different strength, and the power best suited to 
each had to be determined by trial. It was found advisable in 
most cases to regulate it so that the temperature of the electrode 
was below that of a red heat, or such as barely to redden it. 
Of course with the more fusible metals it was necessarily much 
lower than this. The metal is actually volatilized by the dis- 
charge, as is shown by the fact that the characteristic lines of 
its spectrum may be seen with a spectroscope, and the film is 
formed by the condensation of its vapor upon the cooler glass 
surface. For the production of films with brilliant surfaces, 
the strength of the current must not be great enough to give 
the discharge a disruptive character, as this separates some of 
the metal in the form of powder. 
The primary object of the experiments was to obtain films 
of the different metals upon thin pieces of flat glass for the 
purpose of investigating some of their optical characters. The 
to be perfectly successful in its operation, 
and beautiful films of gold, silver, platinum, and bismuth, were 
obtained with ease and certainty. As has been mentioned, it 
seemed probable that the surface of deposit would be dull, but 
the first trial showed that this anticipation was incorrect, and 
the films when removed from the receiver exhibited surfaces 
exquisite perfection and the most brilliant polish. They can 
only compared to the surface of clean liquid mercury, far 
surpassing in luster anything that can be obtained by the ordi- 
nary methods of polishing. 
This circumstance suggested at once a valuable application 
of the process in the production of specula for optical purposes, 
and the subsequent investigations were directed to this end. 
The mirrors first made had been formed upon disks of thin 
glass, such as are commonly used as covers for microscopical 
objects, those being selected which were most free from de- 
fects, and had the best surfaces. By means of a ver delicate 
balance, the weight of the glass disks, both before and 
assay 
after receiving the depo ld be obtained to the one hun- 
tes, hence it was easy to calculate 
ta 
dredth part of a millig 
the thickness of the me 
