58 
THE PRINCIPAL FLOOR. 
If iron is placed in a solution of sulphate of copper, the iron 
is dissolved as sulphate of iron, and copper takes its place. This 
process is termed cementation, and the cup ^exhibited was thus 
produced, the mine-water containing copper sulphate. The 
inscription on the cup is Gott zeigt an Mir sein grose Macht 
der auss Eisen Kupfer Macht , “ God shows in me his great 
power, who out of iron makes copper.” The date of this cup 
is about 1650. 
Some examples of tin plate, with a series illustrating the 
process of manufacture* are exhibited. Tin plate is iron, or steel, 
coated with a thin layer of tin. The iron, which must be 
manufactured with much care, is rolled into sheets of the required 
thinness. Mild steel is now commonly used in place of iron. The 
sheets are cut into rectangular pieces, and these are freed from 
adhering oxide, or any impurities which would inevitably 
prevent the adhesion of the tin, by pickling in a bath of 
sulphuric acid. They are withdrawn, and after washing, are 
annealed in the furnace. The plates are passed through cold 
iron cylindrical rollers, and then, after another annealing, are 
exposed to the action of dilute sulphuric acid, until they become 
perfectly bright. Being cleaned off, the plates are passed 
through hot palm-oil, and then plunged into melted tin, 
covered with grease* or ?with zinc chloride, and the surface 
thus becomes alloyed. The plates are subsequently dipped into 
another pot of metal, then passed between steel rollers, and 
finally cleaned off and polished. Terne plate is coated with 
an alloy of tin and lead, instead of with pure tin. 
Electro-Metallurgy. — Case No. 32. 
Large Electrotype Busts — Nos . 5 and 45 ( near the Stairs 
to Gallery on each side). 
The earliest discovery of the process of electro-metallurgy, was 
announced to the public on the 4th of May 1839, by Professor 
Jacobi* of St. Petersburg. On the 8th of May, Mr. Spencer 
announced to the Liverpool Polytechnic Institution his discovery; 
and on the 22nd of the same month Mr. C. J. Jordan published 
in the Mechanics’ Magazine a description of his method. Some 
beautiful exampes of electro-deposited metals are collected in 
this case, and several others are placed on the neighbouring 
walls. Many surfaces, such as clay, plaster of Paris* wax, &c., 
are not conductors of electricity, and consequently upon these 
metal cannot be precipitated. The late Mr. Robert Murray 
discovered that black lead {plumbago ), rubbed over such articles 
gave them at once a conducting surface, and rendered them fit 
for receiving, by the voltaic battery, a metallic precipitate. 
Thus are formed several of the objects exhibited. 
If silver or gold is to be deposited* preparations of these 
metals are dissolved in cyanide of potassium or some such salt. 
Th$ article is immersed in this, and when connected with the 
