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Medium Mean weight in Specific | Weight of
through which | No- Weight air and gravity single drop in the mercury | di. s of drops. relative size of of respective dropped. ps. single drop. medium. medium.
0.59822 0-864 0-56014
0.43497 0-863 0.40715
2 N 2
these two ingredients were present. "We could scarcely approach to a answer by any of the means hitherto employed. The specific gravities o the two liquids are so close ("864, "863) that the density of the mixtra ■would give us no substantial aid. Though there is a considerable diff« ence (80° C.) in their boiling-points, no one who is familiar with the diffi culties of fractional distillation would place any reliance upon a quantita tive separation based upon volatility. Their refractive indices are nearlj the same*. Their vapour-densities, 2-77, 476, though comparative]] different, are not absolutely very wide apart. They are active and passin towards most of the same chemical reagents, and interfere with one another' reactions. If we have recourse to chemical analysis (Cla H^, CM H16), a ver small experimental error would poiut to a great difference in the proportiw of the two.
To find how far the stalagmomcter (Plate V. fig. 7) is applicable in thicase, it was filled with five liquids in succession:—
1st, with benzol =B.
2nd, with two volumes benzol and one of turpentol =BT.
5th, with turpentol =T.
The time-growth being brought in each case to 5", the number of drops of water required to fill a given volume was counted, allowance being niaie for the meniscus.
Through Air. T. BTa. BT. BaT. B.
102 51 38 34 31 14
102 51 37 33 31 14
101 50 38 33 31 14
.. 49_ ..
Mean.... 1017 502 377 33-3 31 14
Hence a difference of 16-6 per cent, in one of the constituents corresponds to an observed difference, under the most unfavourable condition, of three drops. In other words, the stalagmometer is sensitive to an alteration of about 6 per cent. By increasing the capacity of the recipient, it is clear that the drop-numbers, and therefore their differences, might be increased at pleasure. Thus by counting the number of drops necessary to fill a volume six times the size, we could tell to within one per cent, how much turpentol and how much benzol were present.
But it is perhaps in the cases of the still more proximate identity of isomeric bodies mentioned above that the stalagmometer may be used rather
* The refractive index of turpentol is 1-476; that of benzol does not appear to h»rc been measured; but that it is almost identical with that of turpentol is seen on mixing the two. In those cases in which I propose chiefly to use the stalagmometer, name'.r with isomeric liquids, the method of refraction is useless, because isomeric liquids seem always to have the same refractive indices.
s a stalagmoscope, to render evident rather than to measure a difference f drop-size.
From Table XVI. we gather the general law concerning three liquids, wo of which are insoluble in the third. If a liquid, A, drop downwards nder like conditions in succession through two liquids, B and C, then its 'rop-aize through any mixture of B and C is intermediate between its 'rop-sise through B and its drop-size through C; and the greater the
proportion of p in the mixture, the more nearly does the drop-size of A.
'hrough the mixture approach to the drop-size of A through p alone.
"We have already examined the influences on the drop-size in the case 5LG of the density of the dropping liquid, and of its persistent and stubborn cohesions respectively. Increase in the former two tends to diminish the drop-size; increase in the last to increase it. Let us examine in like manner the influence of the similar properties of the medium.
1. The density of the medium.—Increase in the density of the medium is equivalent to diminution in the density of the dropping liquid, and must therefore be followed by a tendency to increase in the drop-size.
2. Stubborn cohesion of medium.—The resistance to displacement, or stubborn cohesion of the medium, tends to keep back the drop in its place, and makes it necessary for a larger quantity of the dropping liquid to accumulate; that is, it increases the drop-size.
3. Retentive, cohesion of medium.—The same force of persistent or retentive cohesion which causes a drop of a liquid to take the spherical form, would also cause the liquid to give or tend to give a spherical form to an irregularly-shaped volume of a solid, liquid, gas, or vacuum in it. Thus gas-bubbles in liquids have an approximately spherical form, not by reason of the cohesion of the parts of the gas, but by the persistent cohesion of the liquid medium which moulds the gas into that form by which the cohesiou of the liquid is most gratified. Hence increase in the retentive or persistent cohesion of the medium tends to diminish the drop-size of the dropping liquid.
In all cases of SLL we may represent the direction of the influence of the determinants by the following scheme, in which the sign -f denotes a tendency to increase, the sign — one to diminish the drop-size:—
Stubborn cohesion. Persistent cohesion.
This scheme is" verified by the experimental results obtained. Of d liquids water forms the largest drops in falling through air, because in i the Btubborn cohesion prevails to the greatest degree over the joint actio: of persistent cohesion and weight. In water mercury forms drops greats than in all other liquids, because in water (as a medium) stubborn cohesior and weight prevail to the greatest degree over persistent cohesion.
The case SLL may be inverted if the drop-forming liquid be specificallT lighter than the medium liquid. Thus every case of SLL which we hat> examined in which a liquid, A, drops downwards through a liquid, B, has a counter case in which the liquid B drops upwards* through the liquid A.
In order to measure the size of such ascending drops, the stalagmometer (fig. 7) is modified in form. It is not found possible to cause the dropping liquid to adhere with sufficient completeness and uniformity to a solid sphere immersed in the denser medium, in the cases experimented on. The end of the siphon A was turned upwards, and served as the solid whence the liquid dropped, without the interposition of a sphere or other solid. The measuring-tube D was removed from the neck of the cup C, a stopper being inserted in its place. The cup C was filled with water, and the measuring-tube D, being also filled with water, was inverted into it and supported by the holder H. The modified stalagmometer is seen in Plate Y. fig. 10. Care was taken that the end of the siphon A should always be at the same depth beneath the surface of the water in C.
The drop-sizes of the liquids of Table XVI. were first examined by this stalagmometer.
The following Table XVII. shows the number of drops of the various liquids, dropping through water, required to fill the measuring-tube up to the given mark. The measuring-tube employed was different from that used in forming Table XVI. On this account, and because the delivering solid was quite different in shape, and gt only 2", no immediate comparison can be made between Tables XVII. and XVI. In Table XVII. correction is made for meniscus.
We gather from this Table a law quite similar to that deduced from the measurement of the size of the downwards moving drops of water through these same liquids. It is as follows :—
The drop-size of any mixture of two liquids, A and B, dropping up
* See Part I. Introduction.