446 



Prof. Guthrie on Drops, 



[Recess, 



SLL, from a solid a liquid drops through a liquid. 

 SLG, from a solid a liquid drops through a gas. 

 LLL, from a liquid a liquid drops through a liquid. 



Of these three cases, two, SLL and LLL, may be inverted ; that is, the 

 motion of the drop may be towards or from the earth. The gravitation 

 of the drop may be greater than and overcome the gravitation of the 

 medium, the drop descends ; or the gravitation of the medium may over- 

 come that of the drop, the drop ascends. The case SLG cannot be in- 

 verted, because, at all events, at the same pressure every known gas is 

 lighter than every known liquid. 



It will be convenient to consider the case SLG first, because instances 

 of it come more frequently under our notice than of the other two, and 

 because it will be convenient to consider together those cases which are 

 capable of inversion. 



As we are considering the physical aspect of the question, we will only 

 discuss those cases where no chemical action takes place between the terms, 

 and where either no solution takes place, or where it is so small as to be 

 negligible, or of such a kind as to admit of experimental elimination. This 

 limitation of course excludes a vast number of combinations, but it must 

 be made in order to study the purely physical and definite influences which 

 determine the size of a drop. 



SLG. From a Solid a Liquid drops through a Gas, 

 The variable factors are ^ 



1. The self-attraction and cohesion of the liquid : 



A. Dependent on its purely chemical constitution. 



B. Dependent on the proportion and physical relation between its 



heterogeneous parts, when a mixture. 



C. Dependent on temperature. 



2. The adhesion between the solid and the liquid : 

 A, B, C as in 1. 



D. Dependent upon the shape of the soHd. 



3. The adhesion of the gas to the solid. 



4. The adhesion of the gas to the liquid. 



The factors 3 and 4 may be neglected, as we shall at present only con- 

 sider the case where the gaseous medium is air at the ordinary barometric 

 pressure. 



One of these factors, namely temperature, though varying in different 

 cases, may be supposed in the same case to be the same for the different 

 kinds of matter present. Another factor in the same predicament is the 

 locally constant gravitation at the place where the dropping occurs. Lastly, 

 a condition of great influence is the length of the time-interval between the 

 successive drops. This interval we shall call, for brevity, the growth-time, 

 and denote by gt. 



If the above conditions are exhaustive, we may assert that a drop of 



