THE PREPARATION OF CULTURE MEDIA 17 



parency. As agar-agar lias a mucli higher melting-point 

 than gelatine, it is extremely useful in the case of culti- 

 vations which require to be kept at a high temperature ; 

 but in consequence of the separation of water which 

 takes place on its surface after it has congealed, it does 

 not lend itself so satisfactorily to plate cultivations, as 

 1 6 the colonies are apt to run into one another. 



After sterilising it is a good plan to allow the tubes 

 to cool in an oblique position, as in this way a larger 

 surface is obtained, and the liquid which separates out 

 collects at the bottom of the tube, and does not inter- 

 fere ^\dth the growth of the cultivation. Glycerin agar 

 is made by the addition of from 5 to 8 per cent, of 

 glj^cerin after the filtration of the jelly. 



Silica jelly. — A special jelly has recently been 

 devised (W. Kuhne, ' Zeitsch. f. Biol.,' vol. ix. p. 173) 

 to meet the requirements of some refractory organisms 

 — like those of nitrification — which refuse to grow on 

 gelatine, and demand a medium free from organic 

 matter. In this preparation, which is wholly destitute 

 of organic matter, the gelatinous consistency is obtained 

 by means of dialyzed silicic acid.^ To one volume of 

 the sterile solution of dialyzed silicic acid placed in a 

 sterile glass dish with flat bottom one-third to one-half 



^ The dialyzed silicic acid is best prepared by taking a solution of 

 sodium or potassium silicate and pouring this into an excess of dilute 

 hydrochloric acid ; this mixture is then placed in a dialyzer, and the out- 

 side of the latter is kept surrounded with running water during the first 

 day, and subsequently with distilled water, which should be frequently 

 changed until it yields no trace of turbidity with silver nitrate, showmg 

 that the whole of the chlorides have been extracted. The contents of the 

 dialyzer, which, if the solution of alkaline ' sihcate originally employed 

 was not too strong, will be quite clear, is then poured into a flask and 

 concentrated by boiling until it is of such a strength that it is found to 

 readily gelatinise on mixing with the saline solution given above. This 

 solution of silicic acid can then be preserved sterile and ready for use in a 

 flask plugged with cotton-wool in the ordinary way. (For fiu:ther par- 

 ticulars see Winogradsky, Annates de Vlnstitut Pasteur^ vol. v. 1891, p. 97.) 



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