[Illustration: PIPETTE FOR TAKING THE DENSITY OF LIQUIDS.]
For taking the density of a liquid, we plunge the end, B, into it, and
then suck, and afterward close the rubber tube with the clamp. It is
essential that this latter shall hold well, so that the levels may
remain constant.
We now do the reading. Suppose, for example, we read 250.3 mm. on the
pipette, and 147.7 mm. and 152 mm. on the branches of the gauge.
Having these data, we loosen the clamp, and allow the liquid to flow.
On account of capillarity, there remains a drop in B; and of this we
read the height, say 6 mm. A height 250.5 mm - 6 = 244.5 mm. of liquid
raised is, then, balanced by a column of water of 147.5 + 152 = 299
mm.
Now the heights of these two liquids is in the inverse ratio of their
densities:
_d_ 299.5
--- = -----, whence _d_ = 1.22.
1 244.5
We obtain _d_ by a simple division.
When the instrument has been carefully graduated, and has been
constructed by an expert, the accuracy of the first two decimals may
be relied upon. With a little practice in estimating the last drop, we
may, in trying to estimate the density of water, even reach a closer
approximation. In order to measure the height of the drop accurately,
one should read the maximum height to which the liquid rises between
the fall of two drops at the moment when the last ones are falling,
since at that moment, and only at that, can it be ascertained that the
lower level of the bubble is plane.
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