658
THEORY AND PRACTICE OF ENGINEERING.
Rook II.
and afterwards the silver is burnished. A plate of silver is sometimes applied to a plate ofcopper, and beaten or drawn out. brass is silvered by a mixture of chloride of silver,chalk, and pearlash ; when the metal is thoroughly cleaned, the above mixture, moistenedwith water, is rubbed over it: clock dials and scales for thermometers are silvered in thismanner.
Gold is found native and in a metallic state ; it is either massive or crystallised in cubesand octohedra; it melts at a bright red heat, 2016°, and when fused is of a brilliantgreen colour; its specific gravity is 19-3. It is the most malleable and ductile of themetals, and is not tarnished or affected by the action of either air or moisture. It isseparated from copper by cupellation, and from silver by nitric or sulphuric acids. Goldis so malleable that it may be beaten into leaves two hundred and eighty-two thousandthsof an inch in thickness, and a grain of it may be made to cover 56 inches of surface; itsductility is so great, that the same quantity may be drawn out into 500 feet in length.
The Protoxide of Gold consists of
Gold
- 1
-
200
-
96*25
Oxygen
- 1
-
8
-
3*75
1
208
ioo-oo
The Peroxide consists of
Gold
- 1
_
200
_
89*5
Oxygen
- 3
-
24
■
10*5
1
224
100 0
The Perchloride of Gold is decomposable at a red heat, and is soluble in water, alcoholand sulphuric ether; the latter solution is used for gilding. The aqueous solution, ormuriate of gold, is decomposed by several vegetable acids.
Protochloride of tin, added to a dilute solution of chloride of gold, occasions a change ofcolour; the purple powder produced from the compound is used in enamel and glasspainting to produce a fine red colour.
Gold
-
- 1
_
200
-
65
Chlorine
-
- 3
-
108
-
35
1
I coo
100
Platinum resembles silver, but has a less brilliant lustre; it is dissolved by chlorine andnitro-muriatic acid; its affinity for oxygen is extremely feeble; its specific gravity is21-5. In a minute state of division it promotes the union of the hydrogen and oxygengases, absorbing them in large quantities.
Protoxide of Platinum.
Platinum -
-
- 1
.
96
-
92*31
Oxygen
-
- 1
-
8
-
7*69
1
104
100*00
Palladium, Rhodium , Osmium , and Iridium , have all been obtained from the ores ofplatinum; but they have not perhaps been either of them perfectly examined.
Having described the composition of several minerals that compose the earth’s crust, itis necessary to examine somewhat more in detail the methods adopted in extracting theores of lead, iron, copper, and tin, and their conversion into a state to render them useful tothe engineer. The report of the commissioners appointed to examine the mines, publishedin 1842, affords information of a highly interesting character, and which we have takenchiefly for our guide upon this subject.
Lead is seldom or ever found in a pure state, but in combination with carbonic, sulphuric,phosphoric, arsenic, chromic, and other acids, together with oxygen and chlorine. It has acolour of a bluish white, is flexible and soft, melts at a temperature of 612°, and when airis admitted freely with heat it is converted into an oxide. At common temperatures airand water act slowly upon it; its specific gravity is 11 '4.
Oxides of Lead. —When air or water is charged with carbonic or other acids, they willoxidate lead; if the acid be either phosphoric, arsenic, sulphuric, or the liydriodic, aninsoluble crust is formed on the surface of the lead, which protects it, and water may beretained in a cistern or pipes made of it with perfect safety. Carbonic acid in abundanceforms a carbonate, which, instead of encrusting the lead, mixes in a minute state of divisionwith the water, and is highly poisonous.
Protoxide of Lead , Massicot, or yellow lead, is insoluble, fusible at a red heat, and com-bines with many earthy and saline substances; when heated with charcoal it is decomposed,