i8o SURFACE TENSION OF THE MENSTRUUM 



torsion balance a great convenience. The average of fifty or a hundred readings may be 

 checked with the method of Morgan, or Harkins and Brown. In this way much time may be 

 saved without the sacrifice of accuracy. 



Clowes' has shown that it is often desirable to measure the interfacial tension between 

 liquid and oil. Clowes's method consists in immersing the tip of an ordinary stalagmometer 

 beneath the surface of parafiin oil and counting the number of drops in the usual way. This 

 method gives very uniform results. 



The surface tension of ordinary bacterial culture media varies from 57 to 63 dynes per 

 centimeter. The surface tension of standard veal infusion broth is approximately 58 dynes. 

 That of I per cent peptone solution approximately 63 dynes. Since the surface tension of 

 water is 73 dynes, it may be seen that peptone depresses the air-liquid tension about 10 dynes. 



The surface tension of culture media may be raised by treating them with charcoal. 

 This raises the surface tension by removing some of the substances from solution. This 

 treatment removes nutritive elements from the medium. The growth of bacteria^ may like- 

 wise, in some cases, raise the surface tension of the medium in which they are "grown. 



On the other hand, many things may be used to lower the surface tensions of a liquid. 

 Alcohol which has a surface tension of 22 dynes will lower the tension of a fluid in proportion 

 to the percentage added. However, it does not lower the surface tension by being adsorbed 

 into the surface of the liquid, and, therefore, it should not be regarded as a true surface- 

 tension depressant. Soaps, saponin, and many organic compounds are true surface-tension 

 depressants since they are concentrated in the surface of a liquid, and in this way small 

 quantities exert a marked effect in lowering the surface tension. 



A small amount of soluble soap or saponin lowers the surface tension relatively more than 

 larger amounts. The curve expressing such a relationship is that of a parabola. 



For bacteriological work I have found the sodium soaps of the unsaturated fatty acids 

 most satisfactory as surface-tension depressants. Potassium soaps are also very effective, 

 but there is no point in using a potassium soap in a medium containing the sodium ion, since 

 in this case the sodium ion would invariably replace many of the potassium ions, and the final 

 product would be essentially a sodium soap. 



Sodium ricinoleate presents certain advantages over the soaps of other fatty acids from 

 the standpoint of the bacteriologist in that it is very soluble, is readily purified, remains in 

 solution at low temperatures, exerts its maximum action at the pH optimal for the growth of 

 bacteria and production of toxins, and forms perfectly clear solutions. 



It is important to use media which contain no salts of either calcium or magnesium, 

 since these elements form insoluble soaps and would, therefore, remove the latter from solu- 

 tion. 



An important step in preparing glassware — only hard glass should be employed for this 

 work — is to wash it with a hot soap solution in order to remove the surface film usually pres- 

 ent. It is desirable to sterilize the medium and soap solution separately and mix when cool. 



The air-liquid surface tension of water or broth may be lowered to approximately ^2 

 dynes per centimeter by adding a suitable amount of soap of an unsaturated fatty acid. 



In studying the effect of the surface tension of the culture media on bacterial growth, 

 one should not lose sight of the fact that the air-liquid or even the oil-liquid interfacial 

 tension gives no accurate information as to the tension at the interface of the bacteria and 

 water (medium). (Mudd and Mudd-* have studied the effect of interfacial tensions of two 

 phase liquids on bacteria.) 



' Clowes, G. H. A.: Jour. Phys. Cliem., 20, 407. 1916. 



" Larson, W. P., and Evans, R. D.: Proc. Soc. Exp. Biol, and Med., 21, 133. 1923. 



i Mudd, S., and Mudd, B. H.: Jour. Exp. Med., 40, 647. 1924. 



