wed to thicken--either
by leaving the articles in the solution too long or heating the
solution, or having it too strong--it will merely rub off and leave
an irregular surface.

This process succeeds well with yellow brass and Muntz metal, either
cast or rolled, but it does not give quite such uniform (though still
good) results with gun-metal, on which, however, the deposit is darker
and deader in appearance.

A book might be written (several have been written) on the art of
metal colouring, but though doubtless a beautiful and delicate art, it
is of little service in the laboratory. For further information the
reader may consult a work by Hiorns.

Sec. 144. Sieves.

Properly graded sieves with meshes of a reliable size are often of
great use. They should be made out of proper "bolting" cloth, a
beautiful material made for flour-millers. Messrs. Henry Simon and
Company of Manchester have kindly furnished me with the following
table of materials used in flour-milling.

Sieves made of these materials will be found to work much more quickly
and satisfactorily than those made from ordinary muslin or wire gauze.

Relative Bolting Value of Silk, Wire, and Grit Gauze

Threads per inch Trade No. Trade No. Trade No. of
Approximate. of Silk. of Wire. Grit Gauze.

18 0000 18 16

22 000 20 20

28 00 26 26

38 0 32 34

48 1 40 44

52 2 45 50

56 3 50 54

60 4 56 58

64 5 60 60

72 6 64 66

80 7 70 70

84 8 80 80

94 9

106 10

114 11

124 12

130 13

139 14

148 15

156 16

163 17

167 18

170 19

173 20

Sec. 145. Pottery making in the Laboratory.

When large pieces of earthenware of any special design are required,
recourse must be had to a pottery. Small