SURFACE TENSION 



159 



There are a number of ways of evaluating surface tension. 

 The tension in the surface of a liquid can be compared with that 

 in the surface of water, taken as unity. The size of drops falling 

 from a tube depends upon the tension of their surface film; 

 consequently, the size of a drop of liquid of unknown surface- 

 tension value will bear the same relationship to the size of a drop 

 of water (coming from the same opening) as does the tension 

 of the one to the other. This is true provided the liquids weigh 

 the same; if they do not, then their specific gravity 

 must be taken into account. It is more convenient 

 to count drops than to measure their size ; the result 

 is the same, if the total amounts of the known and 

 the unknown liquids are identical. 



The instrument for measuring surface tension by 

 drop counts is known as a stalagmometer (a "drop 

 meter"). It consists simply of a capillary with a 

 flattened mouth and a small reservoir (Fig. 94) and is 

 the invention of the German I. Traube. To him do 

 we owe the initiation of the impetus to surface- 

 tension studies which has gone so far in biological 

 reasoning. 



If 1 cc. of water yields 30 drops, then each drop has 

 a volume of 0.033 cc. If 1 cc. of alcohol yields 105 

 drops, then the volume of each drop is two-sevenths 

 of that of water; therefore, the surface tension of 

 alcohol would be two-sevenths of that of water, pro- 

 vided the two liquids weighed the same; but alcohol 

 is lighter than water, and the weight, not the volume, 

 of a drop is a measure of the tension of its surface film. The 

 following formula evaluates the surface tension of a liquid on 

 the basis of the factors involved when a drop falls from a tube: 



^ 



Fig. 94.— 

 A s t a 1 a g - 

 mometer. 



T = 



n-D-t 



N 



where T is the surface tension of the unknown liquid; n, the 

 number of drops of water; D, the density of the unknown liquid; 

 and t, the surface tension of water. If the tension value of water 

 is taken as unity, then, on the basis of the above formula, with 

 30 drops of water and 105 drops of alcohol and a density of 0.79 

 for alcohol, the surface tension of alcohol becomes 0.226. 



