Chap. 5.]

Forms of Drops.

511

gnats career on the surface of a pond as securely as on land. Someliquids are viscid, and may be drawn into threads; and even water maybe stretched into sheets ere its substance be broken: bubbles producedduring rains, and those pellicles sometimes formed over the mouths ofsmall vials and the interstices of sieves are examples. Water, moreover,in common with other fluids, unites with some substances more readilythan with others. It does not Combine with oils, nor adhere to substancesimpregnated with grease. Hence umbrellas and water-proof dresses aremade of oiled silk ; and rain rolls off the backs of ducks and other aquaticbirds without wetting them, because these fowls dress their feathers withan unctuous fluid which their bodies secrete.

When a vessel contains a liquid that readily unites with it, the liquidStands highest at the edges. Thus in cups of tea or tumblers of water,the fluid climbs up against the sides until it is considerably elevated abovethe general level. This is observable with milk in a pot, pitch in a caul-dron, oil in cans, mercury in vessels lined with an amalgam; melted tinin tinned iron or copper vessels, and fused brass in an iron ladle whoseinterior has been coated with the alloy, as in the process of hard soldering.If, on the other hand, a liquid has no affinity for, or will not unite withthe substance of which the vessel is made, an effect the reverse is pro-duced ; that is, the liquid is depressed at the sides, as when mercury iscontained in a vessel of glass, wood, or earthen wäre ; or even in one ofmetal not lined with an amalgam, or with which the mercury cannot formone. The same thing occurs to fused brass, or lead or tin in crucibles, towater in greasy tubes or dishes, &c.

The same thing, in another form, occurs with drops of liquid. Whenwater is sprinkled on a greasy surface, the particles remain separate how-ever near to each other. By blowing against them, they may be rolledover the plate on which they rest without leaving any portion behind ;but if the substance on which they are dropped combine readily withmoisture their figure is changed ; each becomes flattened by spreading,so that two adjacent drops quickly run together. A drop of oil or speckof grease makes a large stain on a lady’s dress or a marble table. Quick-silver will not unite with marble, but a small portion dropped on a sheetof tin will spread over it like water on damp paper. A portion of tin-men’s solder kept in fusion on clean plates of tin or lead spreads, and isabsorbed in like manner. When ink is spilt upon unsized paper, the lat-ter is stained to a considerable extent: round each drop a broad ring ofmoisture is formed; the darker and grosser particles remaining as anucleus in the centre.

The different forms which drops assume when pendent from solidbodies, are governed by the parts with which they are in contact. Whenwater is sprinkled on a plate partly covered with grease, those particlesthat fall on the clean parts resemble very flat Segments of spheres, whilethose on the greased parts are larger portions of smaller spheres ; theliquid in these swelling out above the base on which they rest, in pre-ference to extending itself like the others upon it. A drop hanging fromthe point of a wire is elongated vertically—if held between the fingerand thumb, it may be stretched out horizontally. If suspended in a ring,its upper surface becomes hollow and its lower one convex, forming aspecies of liquid cup, and supported somewhat like the dishes which che-mists hang over lamps in moveable rings of brass. A drop of liquid in acapillary tube is thus supported ; the tube being nothing more than adeep ring.

The quantity of liquid contained in pendent drops varies with the