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measured the temperature. Fully the upper half of the sphere was coTere ■with cotton-wool, so that the whole of the sphere was kept wet. The cot siderable mass of mercury in the bulb of the dropping sphere or thento meter itself served to make more uniform the temperature of the drop* while the actual contact between the drops and the spherical bulb ensam a tolerably close approximation between the actual temperature of the drop: and that indicated on the stem of the instrument. Although, therefore the temperatures observed cannot pretend to any even approximate posithn accuracy, yet they are certainly in the actual order of magnitude. The arrangement is seen in Plate IV. fig. 6.
In the above Table the temperatures are so grouped together that the means of the groups differ from one another by about 10° C. The single drop-weights are correspondingly grouped, and the mean of each group is then divided by the specific gravity of water (0°= 1) at the mean temperature of the group.
It appears then that, for a range of 20° Centigrade, or 36° F., the difference in drop-size effected by change of temperature in the liquid is inappreciably small, not being more than 0-00277, a quantity almost within the limits of experimental error; for on referring to Table X. we find that the greatest difference between the numbers for glass, which should be equal, amounts to 0*00044 grm., or a sixth of the greatest difference due to variation in temperature.
On the whole, then, we may conclude that the temperature has very little influence on the drop-size in the case of water between the above limits. No doubt, near the point of solidification, where liquids have an incipient
icturc, the drop-size would be subject to sudden changes of magnitude. "ew experiments with other liquids, namely turpentol, acetic acid, and ohol, showed that with them the drop-size was almost equally insensible change of temperature; and in all cases, as with water, the lower the iperature, on the whole, the larger the drop.
We have now examined seriatim all the chief causes upon which the
Dp-size depends in the case SLG. They are, 1. Rate of delivery;
Solids held in solution; 3. Chemical nature of liquid; 4. Geometric
at ion between solid and liquid; 5. Density and chemical nature of solid;
Our data, however, are still insufficient for us to predict, under all cirimstances, the relative sizes of the drops of liquids under known external nulitions. Clearly the missing term is closely related to the specific coesion of the liquid. But what is cohesion ? and how can it be measured? : lies perhaps in the nature of things—it seems at least inevitable—that ic nomenclature of elementary properties should be vague and unsatisfac>ry. The properties of solids—hard, soft, brittle, tough, tenacious, elastic, malleable—do not stand in any definite relation to one another. Even the lardness which resists abrasion, the hardness which resists penetration, the lardness which resists crushing are by no means identical; so that one )ody may possess more of the one sort of hardness than a second body loes, while the second body exceeds the first in another sort of hardness. N'or do any of the above-mentioned properties of solids stand in any simple relation to that resistance to the separation of the contiguous parts which is called cohesion. Thus, by no attribution of this single property of cohesion could we define ice or shell-lac, bodies which are at the same time tough, brittle, elastic, and soft.
We are forced to the conception of two distinct kinds of cohesion—stubborn and persistent. These may coexist, but are not identical. The one is strong to assert, the other pertinacious to maintain. The four following substances may serve to illustrate the possession of these two cohesions in various quantity.
Talc has little stubborn and little persistent cohesion. Glass has much stubborn and little persistent cohesion. Gold has little stubborn and much persistent cohesion. Iron has much stubborn and much persistent cohesion. The necessity for such a discrimination exists in a yet higher degree in liquids. If we conceive two liquids of different nature dropping from the same substance which they both wet, and if there be only one kind of cohesion, the one which has the greatest cohesion will tend most strongly to assume the spherical form; and this would tend to cnuse it to drop sooner, or have a smaller drop-size than the other. On the other hand, the liquid of stronger cohesion will cling most strongly to the film of liquid adhering to the solid; this will keep it longer from falling, and thereby increase its •hop-size. Hence, an increase of cohesion tends to produce two contrary effects. But if there he a similar distinction between the two kinds of < hesion of liquids, as above pointed out in the case of solids, we hare t following consequence. It is the persistent cohesion which causes the i sumption of the spherical form, the stubborn which resists the separate of the drop. The former tends to diminish, the latter to increase its sii As one or other predominates, the size of the drop varies.
Accordingly the drop-size is by no means a measure of what is general called the cohesion of the liquid, but rather a measure of the different between the two cohesions, stubborn and persistent; and the law is, thi the drop-size varies inversely as the persistent, and directly as the stubbor cohesion of the liquid.
In mercury, water, and glycerine the stubborn cohesion is greater i proportion to the persistent cohesion than in the other liquids examined but it by no means follows that persistent cohesion is wanting in mercur or stubborn in alcohol.
When a drop is in the act of falling its stubborn cohesion is in equib' brium with the resultant of two forces—the one, the persistent cohesios tending to produce a spherical form, the other the weight of the drop Since the former of these component forces is, for tlje same liquid, constant it seems as though the weight of the drop might he taken as a measufl and expression of the stubborn cohesion. But such is not the case, becauM we have no ground for supposing that the diameter of the drop where the separation occurs is of constant size; on the contrary, it must be conceded that in larger drops this hypothetical surface of stubborn cohesion is larger than in smaller drops. Further, unless we know the exact shape of a drop in all cases, we are not in a position to deduce the size of the surface of cohesion from the drop-size or drop-weight.
In the cases where it has been tried, it has not been found that the nature of the gaseous medium in the case of SLG exerts any appreciable or definite influence upon the drop-size. Taking glass for the solid and water for the liquid, the medium was changed from air to nitrogen, hydrogen, and carbonic acid. The exceedingly slight variation wrought ih the drop-size by this change may probably have been due to the different solubility of the gases, in water, and the consequent alteration in the cohesion of that liquid.
Having now traced the effect of variation in the conditions which determine the size of a drop in the general case SLG (or where from a solid a liquid drops through a gas), we come to the case SLL (that is, where from a solid a liquid drops through a liquid). As in the cases of SLG, we must here also take the three terms of such chemical nature as to be withont action upon one another.
SLL. From a Solid a Liquid drops through a Liquid.
A prehminary quantitative experiment was made under the following conditions:—Water was made to drop from a glass sphere at the rate
5". The drops were collected in a tube bearing an arbitrary mark.
number of drops required to fill the tube up to this mark was noted.
n the sphere was surrounded by turpentol, and the rate having been jght again* to gt=5", the number of drops of water necessary to fill
tube up to the same mark was counted. The turpentol being replaced benzol, the same operation was performed. The entire arrangement of
Stalagmometerf is seen in Plate V. fig. 7.
£, Y are contrivances described in Part I. for giving a uniform flow of ter.
rhe siphon A rests upon the cotton-wool covering half of the dropping lere and thermometer-bulb G. The sphere is held by its stem B in the mp H. C is half a globular 1 -lb. flask, supported by the filter-stand K. irough the neck of C passes the tube D. C and D are joined liquid;ht by the caoutchouc collar L. A few arbitrary marks are made at E. ae lip of C is turned down to a beak at M above the vessel F. In adjusting the instrument, to get the required value of gt, the holder
is slipped along the table so that the drops from G fall between C and ', and not into D. When the required rate is obtained, it is slipped back :ain. When such liquids as turpentol are used as media, a little water i poured between D and C to protect the caoutchouc. In all cases where
liquid medium is employed C is filled till it runs over. In the first experiment, of which the results are given in the following vable XIII., the numbers are subject to two sources of error. The volume illed is rather small, and no allowance is made for meniscus. In this, as
a all cases of SLL, great care must be taken not to shake the instrument.
There is therefore a greater difference between the drop-sizes of water in benzol and turpentol than between those in turpentol and air. The tur
* A diminution of gt is observed. t EraXay/ids, a drop.
pentol and benzol here employed had the specific gravities of 0-863 asi 0*864 respectively; they may therefore be considered of equal density Hence variation in the liquid medium, independent of variation in its d«a sity, produces an enormous effect upon drop-size. We shall have occasion to return to this case.
The influence which the liquid medium exerts on the drop-size, and tbj share of that influence due to the specific gravity of the medium, will Ih well seen on comparing the drop-sizes of mercury which falls througl various liquid media.
The arrangement of the apparatus for this purpose is seen in Plat* T. fig. 8. As far as A it is similar to fig. 7. The siphon A, fig. 8, is a capillar? tube; its lower end, which is turned vertically downwards, rests upon a sphere of brass, R, which has been washed with nitric acid and sodium-amalgam, and allowed to sonk for some days under mercury. Mercury adheres perfectly to such a sphere. In every case the sphere was immersed just halfway in the liquid. A small capsule S is supported in the liquid on a stand T about half an inch lower than the bottom of the sphere. As soon as gt becomes constantly =5", the vessel V is moved so that S comes under B. Five drops of mercury having been caught, the cup is moved horizontal!'' as before, taken out and replaced by a fresh one, and so on. The batcbw of five drops are washed, dried, and weighed. The results are given in Table XV.
We may, however, previously notice here with advantage a phenomenon which attends the separation of drops under several circumstances, but which can be watched most narrowly in the cases of SLL, because in a liquid the separation of a drop is less abrupt than in a gas.
When water falls from glass through air, immediately after the drop separates, a very minute drop is frequently projected upwards from the upper surface of the drop*. I have not traced the conditions under which this supplementary drop is formed; indeed it is sometimes formed, ami sometimes not, under apparently similar circumstances. No doubt the proximate cause is that the drop at the instant of separation is not spherical; the persistent or retentive cohesion, which brings it almost immediately to its normal shape, does not allow time for its more excentric parts to collect to the main mass; they are therefore by the motion of the main drop flung off and projected upwards.
The same phenomenon is seen much more distinctly when water drops at this rate (y<=5") through benzol or turpentol. In these cases the persistent cohesion of the liquid medium comes also into play.
But the most striking example of supplementary drops is seen when
» The secondary drop may be well shown by holding a plate containing anhydrous cupvic sulphate about two inches below the dropping solid. The white Salt is smootheii«l by pressure under a plate, and its surface, being porous, absorbs the water-drops insUnllv and without splashing. The blue spots of hydrated sulphate show where the faia lias fallen.