2312 Journal of Applied Microscopy 



Forty drops of A. and twenty drops of B. are added to one liter of water in 

 which five centigrams each of sterilized wheat straw and moss has been placed. In 

 place of the wheat straw, 25 gr. of calcium silicate, precipitated and washed, 

 may be added, and the sodium and potassium nitrate can be omitted without 

 any marked effect. 



Marine diatoms can be grown in agar made up with sea water, or the follow- 

 ing synthetic solution : 



Magnesium sulphate - - - - - - - -• gr. 



Magnesium chloride - - - - - - - 4. " 



Distilled water - - - - - - - - 1. It. 



A few drops of solutions A. and B. may also be added with advantage. 



Of course, all the above solutions may be used as liquid cultures, but as there 

 is great difficulty in following a single organism by this method, it is fre- 

 quently best to add either agar or gelatine, as most of the alga^ are readily grown 

 on a solid medium of this kind. Since it is desirable not to have the culture 

 any stiffer than necessary, 0.5 per cent, agar or 5 per cent, gelatine is quite 

 firm enough, if carefully handled. The agar should always be as free from 

 organic matter as possible, this being accomplished by soaking in 1 per. cent, 

 acetic acid for fifteen minutes and then rinsing thoroughly in distilled water. 

 When it is desirable to use a medium absolutely free from organic matter 

 and yet of a jelly-like consistency, there is nothing so satisfactory as the silica 

 jelly recommended by Winogradsky for isolating certain nitrifying bacteria. As 

 the method for making this medium is not generally well understood, most of 

 the published accounts being incorrect or so meagre that it is impossible to 

 manufacture the jelly successfully, the following details are given : 



A volume of sodium silicate (specific gravity, 1.07 to 1.1) is added drop by 

 drop to an equal volume of hydrochloric acid (specific gravity, 1.10), meanwhile 

 keeping the hydrochloric acid well agitated. This mixture is dialized until 

 there is no reaction for chlorine with silver nitrate. A perfectly clear liquid, 

 without the slightest opalescence, that will stand sterilization to 115° C, should 

 result. The reason for the general failure to successfully make this medium is 

 usually due to one of the following causes : 



1. Impure sodium silicate. The soluble glass must be absolutely transpa- 

 rent. If otherwise, it will begin to harden in the dializer, or will be so unstable 

 as to make proper sterilization impossible. 



2. Incorrect specific gravity. This should always be 1.10 for the hydro- 

 chloric acid. The specific gravity of sodium silicate may vary from 1.07 to 1.10, 

 according to the make and consistency used. When commercial soluble glass is 

 bought in liquid form, a specific gravity of 1.07 is about correct. If crystals 

 are used and dissolved in hot water, the specific gravity will vary somewhere 

 between 1.09 and 1.2. 



3. Imperfect dializer tubes. It is absolutely necessary that the dializing 

 membrane be free from holes, and at the same time as thin as possible, to per- 

 mit the most rapid dialization. For this reason the collodion tubes described by 

 by Mr. Kellerman in the Journal of Applied Microscopy and Laboratory 

 Methods (Vol. v, p. 2038, November, 1902) have been used with much sue- 



