460 



Prof. Guthrie on Drops. 



[Recess, 



The experimental numbers obtained are given without omission. The 

 liquids are arranged in the order of magnitude of their drop-sizes. It 

 appears from column 5 (of the specific gravities) that some of the liquids 

 employed were not perfectly pure. This, however, is quite immaterial in 

 the present direction of examination, provided that in all cases where the 

 liquids named are in future employed and compared with those of Table 

 VII., identically the same liquids are meant. 



The numbers of column 6, with which we are now exclusively concerned, 

 present several points of great interest. In the first place, it appears that 

 the specific gravity of a liquid is not by any means the most powerful de- 

 terminant of the drop-size. Thus butyric acid, which has sensibly the 

 same specific gravity as water, gives rise to a drop less than half the size 

 of the water-drop ; while mercury, of singular specific gravity, has no ex- 

 ceptional drop-size. Lastly, it may be observed how that remarkable body 

 water asserts here again its preeminence. The first impression which 

 these numbers make is, that there are three groups of magnitude, n, 2 w, 

 3w. But it is possible that a change in the nature of the solid might 

 throw these drop-sizes into a different order of magnitude ; and certainly 

 until a very much greater number of bodies is examined in this sense, it 

 would be premature to attempt to establish anything like a law. 



It is sufficient for the present to point out that the drop-size is not 

 directly dependent upon either the specific gravity or boiling-point ; nor 

 does it stand in any obvious relation to what is sometimes called the liqui- 

 dity, mobihty, or thinness of a liquid. For we find that glycerine and 

 (from former experiments) cocoa-nut oil both form smaller drops than 

 water, the one being heavier and the other lighter than that bod}'-, and 

 both being viscid or sluggish. On the other hand, alcohol and acetic acid, 

 both perfectly mobile liquids, give rise to drops about half as large as those 

 of glycerine*. 



Hence it is clear that we are still ignorant of that property of a liquid 

 upon which its drop-size mainly depends. We are not yet in a position 

 to connect the drop-size with any of the known physical or chemical pro- 

 perties of liquids. We approach the solution of the problem by studying 

 the effects of change in some others of the variables. 



The adhesion between the liquid which drops and the solid from which 

 it drops is also affected by the curvature and general geometric distribution 

 of the solid at and about its lowest point. And the variation in the adhe- 

 sion between the solid and liquid, caused by the variation in the geometric 

 distribution of the solid, may and does in its turn affect the size of the 

 drop. 



From this aspect, one of the simplest kinds of variation is that offered 



The evaporation of the more volatile of these liquids is a source of slight error ; 

 not so much on account of the direct loss in weight of the drop in falling, as by reason 

 of the cooling which it causes, and the consequent variation in density and adhesion. 

 Such source of variation we shall examine in the sequel, and find insignificant. 



