38 BACTERIA IN RELATION TO PLANT DISEASES. 
. 
experimenting it was found that 5 grams of the clean, white guncotton per 100 
ce. of the fluid gave a solution very satisfactory to work with. About 24 hours is 
required to dissolve the guncotton into a homogeneous mixture, of which there 
should be at least 800 cc. This should be stored in a cork-stoppered bottle of shape 
convenient to hold in one hand. It is then ready for use. ‘The clean test tube, 
thoroughly dry on the inside, is now held in one hand ina slanting position, mouth 
up, while with the other the collodion is poured slowly and steadily into the tube, 
while the latter is slowly rotated. In this way air-bubbles are avoided and the entire 
interior of the tube is moistened. When this has taken place and about an inch of 
fluid has accumulated in the bottom of the tube, the excess is poured back into the 
bottle, slowly rotating the slanted tube, as before, so as to cover again the entire 
interior with as uniform a layer as possible. When the bulk has been poured back, 
the tube is stood upright, mouth down, to drain on a sheet of clean paper. In two 
or three minutes it will have drained sufficiently, the excess of accumulations about 
the mouth being wiped off on the paper now and then. ‘The tube is then seized 
and rotated in a horizontal position for four or five minutes with the mouth in the 
draft of an electric fan, or the rotation may be somewhat longer if no air-current 
is available. A little experience will tell when the sack is dry enough to remove 
from the tube. The strong smell of ether must have somewhat subsided and the 
collodion must not feel wet around the mouth of the tube, as will be the case if the 
layer of collodion is too thick in places. If it is taken out in this condition, the 
thick, wet places will become clouded. ‘The collodion is now cut free at the lips of 
the test-tube by means of a pin-point or other sharp instrument and the tube is filled 
with cool water, taking care to let it also flow between sack and wall of tube if there 
isany shrinkage. Ina minute or two, if the work has been well done, the sack, free 
from air-bubbles and filled with water, may be readily lifted out of the tube. It is 
then placed in a jar of water, where it remains until it is ready to receive the sub- 
stance to be dialyzed. These sacks are quite tough, and there is little danger of 
tearing them during filling and tying. 
When the silicate jelly or other substance has been placed in them, the mouth 
is brought together and tied by means of a small rubber band, the elasticity of which 
keeps the sacks perfectly tight. Silicate jelly should be dialyzed for at least 12 hours, 
and sometimes for 24 hours, if every trace of salt must be removed. ‘The writer 
fills the sacks with the silicate jelly in the afternoon and leaves them in running tap 
water over night. The next morning they are taken out, their contents emptied 
into a clean beaker, the nutrient salts added, and the fluid immediately pipetted into 
tubes, flasks, etc., and sterilized by heat. ‘The nutrient substances should be dis- 
solved in advance, so as not to delay the preparation of the medium. ‘They should 
be added for this purpose to a minimum quantity of water. Some dissolve slowly, 
and there is a preferable order of solution, the glycerin being added last in case of 
Fermi’s solution. 
For the preparation of silicate jelly a Beaumé hydrometer for liquids heavier 
than water is used. C.P. hydrochloric acid of any specific gravity is diluted with 
distilled water until it tests 1.10° on the scale of the hydrometer when cooled 
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