ON DROPS. 



5+5 



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) ; lie 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. 



Let him now substitute a drop of mercury for the milk. By watch- 

 ing the splash very intently he will be able to catch a glimpse of the 

 mercury spread out in the symmetrical, star-like form of f, 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 tolera- 

 bly thickly in the flame of a candle, we get a linely-divided surface of 

 lampblack to which the milk does not adhere any more than the mer- 



FiG. 4. 



cury, 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 rincrs with number- 



VOL. XI. 



