258 H.S. H. WARDLAW. 
were prepared in the form of sacs by covering the inside 
of a test-tube with a layer of a solution of celloidin and 
allowing the sol vent (alcohol-ether) to evaporate off. Before 
the ether and alcohol have completely disappeared from 
the layer of celloidin deposited in this way in the test-tube, 
the latter should be filled with water and the remainder of 
the alcohol and ether dissolved out. Membranes prepared 
by allowing all the ether and alcohol to evaporate off in the 
air are very brittle. The sacs formed in this way do not 
adhere firmly to the inside of the test-tube, and with a 
little patience can easily be coaxed away from the glass. 
These celloidin membranes, when prepared in the right way 
are transparent and flexible. They withstand a consider- 
able tensile stress but are very easily torn. If water be 
poured into the test-tube before enough of the solvent has 
evaporated from the celloidin, the membrane formed will 
be opalescent and will tear so easily as to be useless. A 
suitable solution for the preparation of these membranes 
consists of equal parts of ether and absolute alcohol con- 
taining 5% of celloidin (see Abel, Rowntree and Turner, 
1914). 
The dialysates obtained in these celloidin sacs are per- 
fectly clear when bacterial contamination is avoided. 
When a sac has once been used, however, it is rather 
difficult to clean properly inside, and the dialysate becomes 
infected and turbid in spite of the presence of toluol in the 
surrounding milk. In the later experiments this source of 
contamination was avoided by using a new diffusion sac 
for each dialysis. 
In carrying out the dialyses 25 cc. of water were put in 
the celloidin sac and the latter suspended in one litre of 
milk. It was found that no further change occurred in the 
concentrations of the substances which had diffused through 
into the water after the dialysis had continued for twenty- 
a eee 
