88. Gravity produces different effects upon different substances. This
is commonly expressed by saying that some substances are heavier than
others; for example, I have here a piece of wood and a piece of lead of
equal bulk. The lead is drawn to the earth with a greater force than
the wood. Substances are usually termed heavy when they sink in water,
and light when they float upon it. But a body sinks in water if it
weighs more than an equal bulk of water, and floats if it weigh less.
Hence it is natural to take water as a standard with which the weights
of other substances may be compared.
89. I take a certain volume, say a cubic inch of cast iron such as
this I hold in my hand, and which has been accurately shaped for the
purpose. This cube is heavier than one cubic inch of water, but I shall
find that a certain quantity of water is equal to it in weight; that
is to say, a certain number of cubic inches of water, and it may be
fractional parts of a cubic inch, are precisely of the same weight.
This number is called the _specific gravity of cast iron_.
90. It would be impossible to counterpoise water with the iron without
holding the water in a vessel, and the weight of the vessel must then
be allowed for. I adopt the following plan. I have here a number of
inch cubes of wood (Fig. 26), which would each be lighter than a cubic
inch of water, but I have weighted the wooden cubes by placing grains
of shot into holes bored into the wood. The weight of each cube has
thus been accurately adjusted to be equal to that of a cubic inch of
water. This may be tested by actual weighing. I weigh one of the cubes
and find it to be 252 grains, which is well known to be the weight of a
cubic inch of water.
[Illustration: FIG. 26.]
91. But the cubes may be shown to be identical in weight with the same
bulk of water by a simpler method. One of them placed in water should
have no tendency to sink, since it is not heavier than water, nor on
the other hand, since it is not lighter, should it have any tendency
to float. It should then remain in the water in whatever position
it may be placed. It is difficult to prepare one of these cubes so
accurately that this result should be attained, and it is impossible
to ensure its continuance for any time owing to changes of temperature
and the absorption of water by the wood. We can, however, by a slight
modification, prove that one of these cubes is at all events nearly
equal in weight to the same bulk of water. In Fig. 26 is shown a tall
glass jar filled with a fluid in appearance like plain water, but it is
really composed in the following manner. I first poured into the jar a
very weak solution of salt and water, which partially filled it; I then
poured gently upon this a little pure water, and finally filled up the
jar with water containing a little spirits of wine: the salt and water
is a little heavier than pure water, while the spirit and water is a
little lighter. I take one of the cubes and drop it gently into the
glass; it falls through the spirit and water, and after making a few
oscillations settles itself at rest in the stratum shown in the figure.
This shows that our prepared cube is a little heavier than spirit and
water, and a little lighter than salt and water, and hence we infer
that it must at all events be very near the weight of pure water which
lies between the two. We have also a number of half cubes, quarter
cubes, and half-quarter cubes, which have been similarly prepared to be
of equal weight with an equal bulk of water.
92. We shall now be able to measure the specific gravity of a
substance. In one pan of the scales I place the inch cube of cast iron,
and I find that 7¼ of the wooden cubes, which we may call cubes of
water, will balance it. We therefore say that the specific gravity of
iron is 7¼. The exact number found by more accurate methods is 7·2. It
is often convenient to remember that 23 cubic inches of cast iron weigh
6 lbs., and that therefore one cubic inch weighs very nearly ¼ lb.
93. I have also cubes of brass, lead, and ivory; by counterpoising them
with the cubes of water, we can easily find their specific gravities;
they are shown together with that of cast iron in the following table:—
Substance. Specific Gravity.
Cast Iron 7·2
Brass 8·1
Lead 11·3
Ivory 1·8
94. The mode here adopted of finding specific gravities is entirely
different from the far more accurate methods which are commonly used,
but the explanation of the latter involve more difficult principles
than those we have been considering. Our method rather offers an
explanation of the nature of specific gravity than a good means of
determining it, though, as we have seen, it gives a result sufficiently
near the truth for many purposes.