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° Beaumé 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 ce. 
of distilled water to give a fluid registering 1.07 Beaumé. On adding the fluid 
containing the nutrient salts, and hardening, sodium silicate of sp. gr. 1.07° Beaumé 
gave a rather too fluid medium, and sodium silicate of much higher sp. gr. than 
1.09° Beaumé 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, equal volumes of the two are mixed. 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 may 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 jelly. 
This is made as follows, for this purpose: Freshly-boiled distilled water, 100; 
magnesium sulphate, 0.2; monopotassium phosphate, 1.0; ammonium phosphate, 
10.0. Dissolve. ‘Then add glycerin, 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 open flame (to drive off 
the absorbed air). It is now cooled down to 50° C. and the Fermi added. If this 
has been dissolved over night it must also be brought to a boil and cooled, or have 
the air removed under an air-pump before adding it to the silicate jelly. To 500 cc. 
of the dialyzed fluid, 90 cc. of the Fermi may 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 110°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 trot be opened until the temperature has again fallen to 100° C. It is also 
necessary 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 freshly- 
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 very good success with Fermi for differential purposes. Many organisms grow 
remarkably well on this substratum, while others do not vegetate, or make only a 
scanty growth. 
The observations on this medium are the same as for gelatin or agar. Observe 
character of growth, staining of substratum (green, pink), etc. 
So_ip VEGETABLE SUBSTANCES. 
These should consist of slant cylinders in cotton-plugged test tubes half covered 
with distilled water and steamed 20 minutes at 100° C. on each of three consecutive 
