PREPARATION OF SILICATE JELLY. 39 



to 60° F. Clear homogeneous sodium silicate of any specific gravity is then mixed 

 with distilled water until it is of sp. gr. 1.09° Beaume at 60° F. A great deal of water 

 must usually be added to the sodium silicate, and the first dilution is tedious. For 

 example, 100 cc. of a sodium silicate of sp. gr. 1.42° required the addition of 750 cc. 

 of distilled water to give a fluid registering 1.07 Beaume. On adding the flxiid 

 containing the nutrient salts, and hardening, sodium silicate of sp. gr. 1.07° Beaume 

 gave a rather too fluid medium, and sodium silicate of much higlier sp. gr. than 

 1.09'-' Beaume is apt to set before it has properly dialyzed, or after adding the 

 nutrient .salts and before it can be tubed and slanted. Several liters of the diluted 

 acid and sodium silicate may be conveniently made up at one time. When these 

 are ready, eqxial volumes of the two are mi.xed. This is done by adding the 

 sodium silicate drop by drop to the acid, rather rapidly, stirring meanwhile with a 

 glass rod. The top part of the apparatus shown in fig. 146 ma}^ be used for this 

 purpose. The salty, acid fluid is now ready to be placed in the collodion sacks for 

 dialyzing in running water. It is ready for removal from the water when it is no 

 longer acid to litmus and shows only traces of sodium chloride remaining. An 

 exposure to the running water for 6 hours is scarcely sufficient, unless the sacks are 

 small. 



For many purposes Fermi's solution is a good one to add to the dialyzed jell}-. 

 This is made as follows, for this purpose: Freshly-boiled distilled water, 100; 

 magnesium sulphate, 0.2 ; monopotassium phosphate, i.o ; annnonium phosphate, 

 10. o. Dissolve. Then add gl}-cerin, 45.0. 



The dialyzed silicate jelly is now poured out of the collodion sacks into a clean 

 beaker and brought to a boil for a minute or two over an ojDen flame (to drive off 

 the absorbed air). It is now cooled down to 50'' C. and the Fermi added. If this 

 has beeir dissolved over night it must also be brought to a l)oil and cooled, or have 

 the air removed under an air-pxtmp before adding it to the silicate jelly. To 500 cc. 

 of the dialyzed fluid, 90 cc. of the Fermi ma)- be added. This is stirred with a clean 

 glass rod and then quickly pipetted into test tubes. 



It is now placed in the autoclave without delay in the position desired and 

 heated for 15 minutes at 1 10° C. To avoid tearing the surface of the jelly by steam, 

 the autoclave must be carefully shut steam-tight as soon as the air is driven out, and 

 it must not be opened until the temperature has again fallen to 100° C. It is also 

 necessar}' to keep the autoclave closed on account of loss of ammonia from the 

 ammonium salt. For this reason it is desirable to dissolve the Fermi in freshh- 

 boiled water and to pump out any absorbed air rather than to boil it out. 



Other nutrient salts may be added — Uschinsky's solution, etc. The writer has 

 had \-ery good success with Fermi for differential purposes. Many organisms grow- 

 remarkably well on this substratum, while others do not vegetate, or make onl}- a 

 scanty growth. 



The observations on this medium are the same as for gelatin or agar. Obser\-e 

 character of growth, staining of substratum (green, pink), etc. 



Solid Vegetable Substances, 



These should consist of slant c)'linders in cotton-plugged test tubes half covered 

 with distilled water and steamed 20 minutes at 100° C. on each of three consecutive 



