MINERALOGY. 45 
instrument in the water. Additional weights must be added to bring the 
mark m to the level of the water, and the amount of these weights 
subtracted from the standard weight already referred to, will be the weight 
of the mineral in the air. Call this weight p. Remove the mineral from 
‘the plate and place it in the funnel or hollow cone @; immersed in the 
water the areometer will not sink quite to m, but about to c, the body losing 
in water an amount of weight equal to that of a quantity of water of 
precisely the same volume with itself, or equal to that of the water 
displaced. Additional weights are now to be laid on d until the level m is 
again reached: this amount, which we will call p’, expresses the weight of 
an equal volume of water. We have thus ascertained the weight of 
precisely equal volumes of water and of our mineral; and as water is the 
standard taken, fe will express the ratio of the two, or the specific gravity 
of the body. Thus z:1::p:p', andz ra 
The mineral must also be subjected to the tests afforded by electricity 
and magnetism. A simple instrument for ascertaining whether a mineral 
becomes electrical or not by friction, is represented in pl. 32, fig. 7’. Its 
principal part consists of a horizontally suspended insulated metallic needle. 
Whenever a body becoming electric by friction is presented to the needle, 
this, if in its natural condition, will be attracted: on the contrary, no effect 
will be produced when the body is a conductor. fig. 7* is an instrument 
of similar purpose. At one end is placed a piece of tourmaline. On 
compressing the extremity of this between the fingers, the heat decomposes 
its neutral electricity, the positive passing over to the opposite extremity, 
and thus affecting the electrical state of the needles. The latter is insulated 
by a glass leg. On bringing any electric body near to the end of the 
needle, this will be attracted if the electricity be negative or resinous, and 
repelled if it be positive or vitreous. We are thus enabled to detect, not 
only the existence of free electricity in a body, but also its character. 
Pl. 32, fig. 6, presents a simple arrangement for determining the magnetism 
of a mineral. Here a is a magnetic needle, supported on the point of the 
pivot b, and, when permitted to play freely, taking up a position in the 
magnetic meridian. On bringing a magnetic mineral near to the poles of 
the needle, this will either be attracted or repelled, as the end of the needle 
to which the minera! is presented is of unlike or like character with the 
corresponding part of the latter. Should there be no free magnetism in the 
mineral, there will of course be no deflection of the needle. 
The sensible internal peculiarities of minerals are very numerous. Thus 
a fresh fracture may be produced, and the color and degree of translucency 
or transparency of small chips observed ; the color aiso of the powder may 
be compared with that of the solid mass. The shape of the natural 
fragments is also to be noted. Thus galena breaks into cubes, calcareous 
spar into rhombohedrons, and sulphuret of antimony into pointed crystals. 
In the natural cleavage of different minerals we observe a greater or less 
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