1864.] 



Prof. Guthrie on Drops. 



445 



shall have to consider drops which move upwards*. Drops of this kind 

 are so seldom met with that no distinguishing name has been given to 

 them. We shall find it convenient to include them in the general term 

 drop, though it may appear at first inapplicable to them. 



Without attempting to give an exhaustive definition, it will be sufficient 

 to define a drop as a mass of liquid collected and held together by the 

 attraction of its parts and separated from other matter by the attraction 

 of gravitation. This definition will exclude such drops as those of mist or 

 rain, and will include the upward-moving drops mentioned above. 



It follows that the size of a drop may depend upon and be influenced by 

 variation in — 



(1) The self-attraction and cohesion of the drop-generating liquid ; 



(2) Its adhesion to the matter upon which the drop is formed ; 



(3) The shape of the matter from which the drop moves ; 



(4) The physical relation of the medium through which the drop moves, 

 on the one hand, to the liquid of which the drop is formed, and on the 

 other, to the matter on which it is formed ; 



(5) The attraction of the earth, or gravitation, upon the drop-forming 

 liquid and upon the medium, as influenced by their respective and relative 

 densities, and by variation in the attracting power of the earth. 



In order to study systematically the influence which each of these factors 

 exerts, each must be varied in succession while the others remain con- 

 stant. 



Denoting the three states of matter, solid, liquid, and gaseous, by the 

 symbols S, L, G respectively, and considering the symbols in the order 

 in which they are written to denote respectively the matter from which 

 the dropping takes place, the drop and the medium, we get a convenient 

 notation. 



As we are speaking at present exclusively of liquid drops, L must 

 always hold the middle place in the symbol. 



Of the eight symbolically possible variations, 



(1) (2) (3) (4) (5) (6) (7) (8) 

 SLS, SLL, SLG, LLS, LLL, LLG, GLS, GLG, 

 (I), (4), and (7) are physically impossible on account of the superior 

 cohesion of solids over Hquids, (6) and (8) are physically impossible on 

 account of the superior density of liquids over gases. 



SLL, SLG, and LLL are therefore the only cases we have to consider. 

 That is, 



* Owing to the numerical preponderance of downward-moving drops, we are prone to 

 associate the ideas of " drop " and *' down." How far I may be justified philologically 

 » in using the expression " drop up," must depend upon the relative primitiveness of the 

 noun and verb " drop." Once for all, I beg permission to use the term drop in this more 

 extended sense. 



Of course, in the absence of positive levity, an upward drop can only be caused by the 

 downward motion of the medium in which the drop moves. 



