655
THEORY AND PRACTICE OF ENGINEERING.
Rook II.
The Dichromate of Lead, which contains two of oxide of lead and one of chromic acid,forms valuable pigments, used in oil and water-colours, dyeing, See.
Chromic Acid, which is a compound containing three of oxygen, combines with potassa,mercury, silver, chlorine, fluorine, &c.
Cobalt _This mineral is of a grey colour with a reddish tint, is very heavy and brittle.
It is found native, combined with iron, nickel, arsenic, sulphur, See. Its specific gravity is8-5, and it is used to give a blue colour to glass; it crystallises into cubes, octohedrons,and dodecahedrons.
Oxide of Cobalt , or protoxide, is obtained by heating the carbonate of cobalt out of thecontact of air; it is of a greenish grey colour, and imparts a blue tint to glass andenamel.
Cobalt - - -1 - 30 - 78'9
Oxygen - - 1 8-21-1
1 38 - 1000
Smalt is a blue-coloured vitreous compound, produced by fusing zaffre, which is animpure oxide which remains after the arseniuret of cobalt has been heated to expel thearsenic, the cobalt becoming oxidated ; this is mixed with twice its weight of finely-powdered flint. Smalt and azure blue are formed by fusing zaflre with glass, which ina hot state is dropped into water, and afterwards reduced to a fine powder.
Arsenic is of steel grey colour, brilliant lustre, crystalline texture, brittle, and soontarnishes on exposure to the air. Its specific gravity is 5*8, and it is readily volatilised ata temperature of 360°; and when exposed freely to the air produces arsenious acid.
Arseniotis Acid (white arsenic'). — It is white, semi-transparent, brittle, and becomesopaque by exposure to the air; it condenses into small octohedral crystals. It is solublein sixteen of water at a temperature of 212°, a portion being again deposited in crystals asthe solution cools. It is nearly tasteless, but highly poisonous. When arsenious acid isslowly sublimed, octohedral and tetrahedral crystals are formed, derived from a rhombic
prism.
Arsenic
- I
38
76
Oxygen
- 1} -
12
24
The
arsenious acid combines
with bases and forms the
50
100
Arsenites. — Those of potassa and soda are easily soluble and uncrystallisablc, those oflime, strontia, baryta, and magnesia, are more difficult.
Scheele's Green, a fine apple-green colour and used as a pigment, is a mixture of thisacid with a solution of sulphate of copper. With lead, antimony, and bismuth, white pre-cipitates are formed.
Nickel is a brilliant white metal, both ductile and malleable, and not oxidised by ex-posure to the air or moisture at common temperatures. Its specific gravity is 8-5.
Protoxide of Nickel consists of
Nickel
- 1
28
77-77
Oxygen
- 1
-
8
22-23
1
36
100 00
Nickel combines with chlorine, iodine, fluorine, sulphur, Sec.
Meteoric Stones have a black surface, when broken exhibit a coarse texture and a greycolour; they are compounds of silica, magnesia, iron, and nickel.
Mercury has much the same colour and brilliancy as silver ; it is liquid, forms roundglobules, which when brought in contact readily unite together ; exposed to the air itquickly tarnishes; at 40° it loses its liquid state, and becomes solid and malleable, and inthe frozen state its specific gravity is 15-6. At a temperature of 670° it boils, and becomesvapour; at 60° its specific gravity is 13-5.
Native Cinnabar is the principal ore of mercury, and occurs massive and crystallised insix-sided prisms, rhombs and octohedra. It is sometimes of a steel grey, and at others abright red colour; it is a bi-sulphuret of mercury.
Oxide or Protoxide of Mercury is insoluble in water and muriatic acid, but is dissolvedby acetic acid.