144 C. J. T. DORYLAND 



cate gives us sodium and potassium salts in the final medium 

 instead of only sodium salts, thereby lessening the danger of too 

 great a concentration of sodium salt. 



The detrimental influence of too great a concentration of 

 the sodium and potassium salts can be still further lessened 

 by using a mixture of acids; for example, we may use equiva- 

 lent solutions of HCl, H2SO4 and H3PO4, thus giving in the 

 finished medium chlorides, sulphates and phosphates of both 

 sodium and potassium. Experiment demonstrated that the 

 ''Stevens Temple Method" might be still further modified by 

 eliminating the MgCOs or Na2C03. 



The time of precipitation and gelatinization of H2Si03 de- 

 pends largely on two factors, first, reaction and second, con- 

 centration. Gelatinization is delayed or entirely prevented by 

 either an excess of acid or an excess of the Na2Si03 or K2Si03. 

 The most rapid gelatinization with any concentration takes 

 place in a neutral solution. 



The solid synthetic media were prepared in the following 

 manner: Solutions of HCl, H2SO4 and H3PO4 were each stand- 

 ardized separately against the Na2Si03 and K2Si03 solution, 

 so that 1 cc. of each acid would just neutralize 1 cc. of the sili- 

 cate solution. ■ Whenever an organic acid such as formic, ace- 

 tic, lactic or tannic, etc., was used as a source of energy it was 

 made in sufficient concentration to just neutralize an equal 

 volume of the silicate solution. These acids, that is the HCl, 

 H2SO4, H3PO4, and the organic acid (let us say acetic) were 

 then mixed together in such proportion that the resulting salts 

 from the sodium and potassium silicate would be present in 

 the final sihcate medium in quantities, approximately, inversely 

 proportional to their osmotic action, thus giving a minimum 

 osmotic pressure. Before standardizing the HCl there was 

 added to it 0.5 gram MgS04, 0.01 gram CaCOj or CaO, 0.01 

 gram of Fe2(S04)3 and 0.01 gram of Mn SO4, per hter. Am- 

 monia nitrogen was added to the HCl as ammonium sulphate. 

 Cyanide nitrogen was added to the HCl as potassium ferri- 

 cyanide. Nitrite nitrogen was added to the neutral solution 

 in the Petri dish, nitrate nitrogen was added to the acid mixture 



