280 
PROFESSOR J. A. EWING AND MR. W. ROSENHA1N 
Our first expedient was to surround the specimen with an atmosphere of pure dry 
hydrogen gas, and to examine the surface through a glass or mica window in the 
containing vessel. The hydrogen was prepared in the usual way by the action of 
pure dilute sulphuric acid on pure granulated zinc, and the gas was dried and purified 
by bubbling through several wash-bottles containing pure sulphuric acid, caustic 
potash, and permanganate of potash solutions respectively. As a final precaution, 
it was passed through a long tube of glazed porcelain kept at a red heat and packed 
with clean pieces of sheet-iron. It was a specimen of similar iron that was to be 
experimented upon, and we hoped that after passing over this large quantity of hot 
iron, the hydrogen would be free from any impurities capable of tarnishing the 
polished surface of our specimen. The specimen itself was a strip of sheet-iron, of 
the pure kind used for transformer plates, measuring 3 centims. in length and 
3 to 4 m iH ims. in width. It was bent to a j"™| shape, and the upper flat surface was 
polished and etched. The containing vessel is shown in section in fig. 1. It con- 
Fig. 1. 
sisted of a brass cylinder, A, with a screw-cap, B, provided with a window of thin 
glass or mica. The bottom ol the cylinder was formed by a plug ol slate, 0, fitting 
against a shoulder, and held in place by a screw-collar, I). Through this slate plug- 
passed two stout copper wires, EE ; the upper ends of these rods were split to receive 
the two legs of the specimen F, whose flat, polished surface thus came immediately 
beneath the window in the cap. At GG tubes entered the vessel ; these allowed the 
passage of the current of hydrogen. 
All the joints were very carefully fitted, and made as nearly air-tight as possible ; 
leakage, however, was not an important matter as the hydrogen was kept at a 
