1914] William N. Berg 193 



butyric acid or other organic substance to A and B. As a result of 

 a continued tho unequal rate of combustion between A and B, the 

 concentration of the metabolite might be continually different be- 

 tween the two regions and still cause no surface tension, unless, as 

 a special case, that difference happens to be one between low con- 

 centrations. The point to be emphasized is that surface tension is 

 caused by special and not by general differences in concentration of 

 organic metabolites. 



Of course, there are some organic substances that affect the 

 surface tension of water very little, and consequently no surface 

 tension changes would be brought about in those regions where they 

 are being metabolized. As examples of such substances, tartaric, 

 citric and oxalic acids may be mentioned. Aqueous Solutions of 

 these acids, even of high concentration, differ very little in their 

 surface tension from that of pure water, as indicated by the follow- 

 ing data from Linebarger.^^ 



Per Cent Surface tension: 



ofacid dynes/cm., i5°C. 



Water (pure) 0.00 7127 



Tartaric acid 18.18 71-44 



Tartaric acid 53-32 73-86 



Citric acid 6.12 69.35 



Citric acid 5.08 65.19 



Oxalic acid 1.53 70.65 



Oxalic acid 9.13 69.85 



Two adjacent regions would have practically no surface tension 

 between them if one were water and the other an aqueous Solution 

 of tartaric acid of practically any concentration. The same is true 

 for the other two acids. 



By an accurate method, Morgan and Woodward^^ (p. 1256) 

 found the surface tension of blood and blood serum to be 45.4 dynes 

 per cm. at 37° C. ; their value for water is 69.84. The figures are 

 mentioned here for comparative purposes. 



The third class of Solutions to be considered are the aqueous 

 Solutions of the inorganic salts present in biological Solutions. Can 

 these be regarded as sources of surface energy? This is doubtful 



^1 Linebarger : Jour. Amer. Cheni. Soc, 1898, xx, pp. 128-130. For data on 

 the salts of these acids see Morgan and McKirahan : Jour. Amer. Chem. Soc, 

 1913, XXXV, pp. 1759-1767. 



^2 Morgan and Wood ward : Jour. Amer. Chem. Soc, 1913, xxxv, pp. 124^-1262. 



