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PIPETTE FOR TAKING THE DENSITY OF LIQUIDS.

The accompanying engraving represents a simple apparatus, which any person accustomed to working glass can make for himself, and which permits of quickly, and with close approximation, estimating the density of a liquid.  In addition, it has the advantage of requiring but a very small quantity of the liquid.

It consists simply of a straight pipette, A B, to which is affixed laterally, at the upper part, a small U-shaped water gauge.

The two branches of the gauge, as well as the pipette itself, are graduated into equal divisions.  If need be, the graduating may be done by simply pasting on the glass strips of paper, upon which a graduated scale has been drawn.  The zero of the pipette’s graduation is exactly at the lower extremity, B. The graduation of the two gauge tubes extends in both directions from a zero situated near the center.  The zeros of the two branches must correspond as exactly as possible, so that they shall be in the same horizontal plane when the apparatus is fixed upon a support.  To render the apparatus complete, it only remains to adapt, at A, a rubber tube provided with a wire clamp, and terminating in a short glass tube for sucking through with the mouth.

[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.  The error in such reading does not reach half a millimeter, and, as a suitable height of the apparatus permits of having columns that vary between 13 and 30 centimeters, an error of this kind is but 1-300.  This is the limit of precision of the method.