104 Physico-Chcniical Basis of Striated Muscle Contraction [Sept. 



frequently seen in the literature. This is an example taken from 

 Freundlich's Kapillarchemie, p. 4: 



Stirface energy = surface tension X area of surface. 



A similar Statement is made by Michaelis,^ and others. Nothing 

 further was stated that would enable the reader to use such a 

 formula in making calculations were it desired. Expressing the 

 surface tension in dynes per centimeter and the area of the surface 

 in cm.2, what is the surface energy? The answer is very simple 

 after one has taken the time to look the matter up. After a formula 

 such as the above, a numerical example ought to be given, so that 

 it means more than so many words to the average reader. Suppose 

 it is desired to calculate the amount of energy required to form a 

 water-surface (in contact with air) of i sq. cm. area? Or, what is 

 the same thing, how much energy is liberated or is available for 

 external work when the above water-surface diminishes by i sq. 

 cm.? According to Michaelis (1. c, p. 14), this will require (or 

 liberate) 70 ergs or 7 X lo"'^ kilogram-meters. The method of using 

 the formula to obtain this result, simple as it is, was not given by 

 Michaelis, altho at least one example of the use of a formula is 

 desirable because it will enable the reader to make many other cal- 

 culations. 



Following is a numerical example of the kind mentioned above. 

 How much energy is required to form a water-surface (against 

 air) of I sq. cm.? In the formula it is assumed that the surface 

 tension remains constant during the change in area : 



surface energy required ^surface tension X increase in area, 



or 



surface energy liberated = surface tension X decrease in area, 



(ergs) = (dynes per cm.) X (cm.^). 



Since the surface tension of water-air is about 70 dynes per 



cm., it is evident that-^^ X i cm.2^70 ergs = the amount 



cm. 



of energy required. The erg is a unit of work (or energy) and is 

 the work done when a mass is moved i cm. by a force of i dyne. 



' Michaelis : Dynamik der Oberflächen, p. 13. Dresden, 1909. 



