473 



Prof. Guthrie on Drops. 



[Recess, 



pentol and benzol here employed had the specific gravities of 0-863 and 

 0*864 respectively ; they may therefore be considered of equal density. 

 Hence variation in the liquid medium, independent of variation in its den- 

 sity, produces an enormous effect upon drop-size. "We sliall have occasion 

 to return to this case. 



The influence which the liquid medium exerts on the drop-size, and the 

 share of that influence due to the specific gravity of the medium, will be 

 well seen on comparing the drop-sizes of mercury which falls through 

 various liquid media. 



The arrangement of the apparatus for this purpose is seen in Plate Y. 

 fig. 8. x\s far as A it is similar to fig. 7. The siphon A, fig. 8, is a capillary 

 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 soak 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 R. 

 Five drops of mercury having been caught, the cup is moved horizontally 

 as before, taken out and replaced by a fresh one, and so on. The batches 

 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, and 

 sometimes not, under apparently similar circumstances. No doubt the 

 proximate cause is that the drop at the instant of separation is not sphe- 

 rical ; the persistent or retentive cohesion, which brings it almost imme- 

 diately 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 (fft = 5") through benzol or turpentol. In these cases the per- 

 sistent 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 sliown by holding a plate containing anhydrous 

 cupric sulphate about two inches below the dropping solid. The white salt is smoothened 

 by pressure under a plate, and its surface, being poroUs, absorbs the water-drops instantly 

 and without splashing. The blue spots of hydrated sulphate show where the water 

 has fallen. 



