1864.] 



Prof. Guthrie on Drops. 



469 



structure, the drop-size would be subject to sudden changes of magnitude. 

 A few experiments with other liquids, namely turpentol, acetic acid, and 

 alcohol, showed that with them the drop-size was almost equally insensible 

 to change of temperature ; and in all cases, as with water, the lower the 

 temperature, on the whole, the larger the drop. 



We have now examined seriatim all the chief causes upon which the 

 drop-size depends in the case SLG. They are, 1. Rate of delivery; 

 2. Solids held in solution; 3. Chemical nature of liquid; 4. Geometric 

 relation between solid and liquid ; 5. Density and chemical nature of solid ; 

 6. Temperature. 



Our data, however, are still insufficient for us to predict, under all cir- 

 cumstances, the relative sizes of the drops of liquids under known external 

 conditions. Clearly the missing term is closely related to the specific co- 

 hesion of the liquid. But what is cohesion ? and how can it be measured ? 

 It lies perhaps in the nature of things — it seems at least inevitable — that 

 the nomenclature of elementary properties should be vague and unsatisfac- 

 tory. The properties of solids — hard, soft, brittle, tough, tenacious, elastic, 

 malleable — do not stand in any definite relation to one another. Even the 

 hardness which resists abrasion, the hardness which resists penetration, the 

 hardness which resists crushing are by no means identical ; so that one 

 body may possess more of the one sort of hardness than a second body 

 does, while the second body exceeds the first in another sort of hardness. 

 Nor do any of the above-mentioned properties of solids stand in any simple 

 relation to that resistance to the separation of the contiguous parts which 

 is called cohesion. Thus, by no attribution of this single property of co- 

 hesion could we define ice or shell-lac, bodies which are at the same time 

 tough, brittle, elastic, and soft. 



We are forced to the conception of two distinct kinds of cohesion — stub- 

 horn and persistent. These may coexist, but are not identical. The one 

 is strong to assert, the other pertinacious to maintain. The four following 

 substances may serve to illustrate the possession of these two cohesions in 

 various quantity. 



Talc has little stubborn and little persistent cohesion. 



Glass has much stubborn and little persistent cohesion. 



Gold has little stubborn and much persistent cohesion. 



Iron has much stubborn and much persistent cohesion. 



The necessity for such a discrimination exists in a yet higher degree in 

 liquids. If we conceive two liquids of diiferent nature dropping from the 

 same substance which they both wet, and if there be only one kind of co- 

 hesion, the one which has the greatest cohesion will tend most strongly to 

 assume the spherical form ; and this would tend to cause it to drop sooner, 

 or have a smaller drop-size than the other. On the other hand, the Hquid 

 of stronger cohesion will cling most strongly to the film of liquid adhering 

 to the solid ; this will keep it longer from falling, and thereby increase its 

 drop-size. Hence an increase of cohesion tends to produce two contrary 



