Page:Popular Science Monthly Volume 11.djvu/563

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ON DROPS.

ter, on a smooth dark surface of wood or paper, from a height of, say, six inches (milk is better than water, as it is easier to see, especially on a dark ground); he will observe that the liquid makes a blot with a more or less regular undulated edge, but the splash is too quick to follow with the eye.

PSM V11 D563 Electricity and milk 1.jpg
Fig. 3.

Let him now substitute a drop of mercury for the milk. By watching the splash very intently he will be able to catch a glimpse of the mercury spread out in the symmetrical, star-like form of e, Fig. 9. After the drop has been thus spread out it recovers its globular form, since the mercury does not wet the plate. On increasing the height of fall a few inches, it will be noticed that small drops split off in a more or less complete circle, and are left lying on the plate, while the rest of the drop gathers itself together in the middle of the circle.

The chief reason why these appearances could not be seen with milk is, that the milk wets the glass or wood and sticks to it, while the mercury does not. But by smoking a slip of glass or card tolerably thickly in the flame of a candle, we get a finely-divided surface of lampblack to which the milk does not adhere any more than the mercury,

PSM V11 D563 Electricity and milk 2.jpg
Fig. 4.

and by very careful watching we may notice that the same radial star is formed by the milk, but it is much more difficult to catch sight of than the mercury-star. But if the mark on the lamp-black be examined after the drop of milk or mercury has rolled away, it will be found to consist of delicate concentric rings with number-