slow, though preventing a vacuum.
ELECTROPLATING.
Electroplating is the art of coating metals with metals by means of electricity. Silver, copper, and nickel are the metals most generally deposited. The article to be coated is suspended in a chemical solution of the metal to be deposited. Fig. 84 shows a very simple plating outfit. A is a battery; B a vessel containing, say, an acidulated solution of sulphate of copper. A spoon, S, hanging in this from a glass rod, R, is connected with the zinc or negative element, Z, of the battery, and a plate of copper, P, with the positive element, C. Current flows in the direction shown by the arrows, from Z to C, C to P, P to S, S to Z. The copper deposited from the solution on the spoon is replaced by gradual dissolution of the plate, so that the latter serves a double purpose.
In silver plating, P is of silver, and the solution one of cyanide of potassium and silver salts. Where nickel or silver has to be deposited on iron, the article is often given a preliminary coating of copper, as iron does not make a good junction with either of the first two metals, but has an affinity for copper.
Chapter X.
RAILWAY BRAKES.
The Vacuum Automatic brake—The Westinghouse air-brake.
In the early days of the railway, the pulling up of a train necessitated the shutting off of steam while the stopping-place was still a great distance away. The train gradually lost its velocity, the process being hastened to a comparatively small degree by the screw-down brakes on the engine and guard's van. The goods train of to-day in many cases still observes this practice, long obsolete in passenger traffic.
An advance was made when a chain, running