November 7, 1919] 



SCIENCE 



439 



edge of any approximate values for the radii; 

 and this constitutes one of the chief reasons 

 for expecting these tables to prove generally 

 serviceable. 



The authors deserve the thanks of optical 

 computers further, in particular, for their 

 care in testing results by trigonometrical cal- 

 culations. Judging from more than a hun- 

 dred such verifications, they inform us, the 

 small errors in the approximate values of 

 spherical aberration occur only in the fourth 

 decimal place, so that they would hardly in- 

 fluence the specifications to be given to the 

 mechanician. The data in the first tables 

 run to three decimal places. 



Of major significance are the graphs, pages 

 80 and 81, showing the performance of 

 typical lenses of the various types at different 

 apertures. Group A makes quite the best 

 showing. The final page, with some general 

 conclusions, may well be read first. 



American readers will have noticed already, 

 from certain reports published by the Bureau 

 of Standards, that projects not wholly dis- 

 similar to this have been under consideration, 

 and are already partially realized, for lighten- 

 ing the arduous labor of finding satisfactory 

 first approximations in definite types of lens 

 design. Henky S. White 



Bureau op Standards 



SPECIAL ARTICLES 



ELECTROLYTES AND COLLOIDS 



The effect of ions on the physical properties 

 of proteins is one of the most interesting 

 chapters of colloid chemistry. The work on 

 this topic quoted in the textbooks of colloid 

 chemistry suffers from two sources of error, 

 namely, first, that the effect of the hydrogen 

 ion concentration is generally ignored, and 

 second, that the effect of the nature of ions 

 on the physical properties of proteins is often 

 ascertained in the presence of an excess of an 

 electrolj'te. Proteins are amphoteric electro- 

 lytes and therefore occur in three states 

 according to the hydrogen ion concentration, 

 namely as : (1) protein, free from ionogenic 

 impurities, (isoelectric protein) ; (2) metal 

 proteinates, e. g., sodium proteinate or cal- 



cium proteinate, etc.; and (3) protein acids, 

 e. g., protein chloride or protein sulfate, etc. 

 For gelatin the hydrogen ion concentration 

 defining the isoelectric point is, as Michaelis'- 

 first showed, about 2 X 10'' ^ (or in Soren- 

 sen's logarithmic symbol pH^4.7). At this 

 hydrogen ion concentration gelatin can prac- 

 tically combine with neither anions nor cations 

 of an electrolyte. "When the hydrogen ion 

 concentration becomes lower than 2 X 10"^> 

 e. g., through the addition of ISTaOH, part of 

 the isoelectric gelatin is transformed into 

 sodium gelatinate, and the relative amount of 

 isoelectric or non-ionogenic gelatin trans- 

 formed into sodium gelatinate increases with 

 the diminution of the hydrogen ion concen- 

 tration. Sodium gelatinate can exchange its 

 cation with the cation of neutral salts but 

 is not (or practically not) affected by the 

 anion of a neutral salt. When we raise the 

 hydrogen ion concentration of gelatin solu- 

 tions above that of the isoelectric point, e. g., 

 by adding HCl, isoelectric gelatin will be 

 transformed into gelatin chloride and the 

 transformation will become the more com- 

 plete the higher the hydrogen ion concentra- 

 tion, until finally all the isoelectric gelatin is 

 transformed into gelatin chloride. The gela- 

 tin-acid salts can exchange their anion with 

 the anion of other salts but are not (or prac- 

 tically not) affected by the cation of other 

 salts. - 



While isoelectric gelatin has a minimal 

 osmotic pressure, a minimal power of swell- 

 ing, a minimal viscosity, a minimal trans- 

 parency, a minimal alcohol number, etc., 

 gelatin salts, e. g., sodiiim gelatinate or 

 gelatin chloride, have a high osmotic pres- 

 sure, a high power of swelling, a high vis- 

 cosity, etc. The writer has been able to show 

 by volumetric analysis that the osmotic pres- 

 sure, the power of swelling, etc., of gelatin 

 increase with the relative amount of isoelec- 

 tric gelatin transformed into gelatin salt.^ 

 The physical properties of gelatin, e. g., its 



1 Michaelis, L., "Die Wasserstofiionenkonzentra- 

 tion," Berlin, 1914. 



2 Loeb, J., J. Gen. Physiol, 1918-19, L, 39, 237. 



3 Loeb, J., J. Gen. Physiol., 1918-19, I., 237, 363, 

 483, 559. 



