. Laboratory work in bacteriology. Bacteriology. COL^ODIUM SACS. 497 be used in making sacs. This is notably true of celloidin solutions. The film of coUodium must possess a certain degree of elasticity, otherwise it cannot be slipped off the tube on which it is deposited. The tubes should be preferably made of yellow glass and should be about 320 mm. long. The end should be perfectly rounded off, like that of a test-tube. These tubes can be obtained having a diameter of 14, 18, 25 or 30 mm. The 18 mm. tube is the most con- venient one for ordinary Fig. 70. The rolling of coUodium s
. Laboratory work in bacteriology. Bacteriology. COL^ODIUM SACS. 497 be used in making sacs. This is notably true of celloidin solutions. The film of coUodium must possess a certain degree of elasticity, otherwise it cannot be slipped off the tube on which it is deposited. The tubes should be preferably made of yellow glass and should be about 320 mm. long. The end should be perfectly rounded off, like that of a test-tube. These tubes can be obtained having a diameter of 14, 18, 25 or 30 mm. The 18 mm. tube is the most con- venient one for ordinary Fig. 70. The rolling of coUodium sacs. The beaker contains three sacs. The coUodium is placed in a small glass cylinder, about 4 cm. in diameter and 9 cm. high. This is inclined in the manner indicated in Fig. 70. The clean, dry glass tube is inserted into the coUodium and and slowly rotated. Care must be taken to avoid touching the walls of the cylinder. The tube may be rested, at the desired angle, on the lower jaw of a retort clamp. From time to time, the tube is with- drawn almost completely out of the coUodium and rolled in the air. When the tube is returned to the liquid another coat of coUodium is deposited. By repeating this several times a good layer of coUodium is deposited on the glass. It may happen that while the coUodium is of the proper thickness above, where it has been repeatedly ex- posed to the air, yet only a very thin layer will cover the rounded end of the tube since this has remained continuously in the liquid. This' difBculty can be overcome by completely withdrawing the tube out of the coUodium, rotating a few moments in the air and then returning it slowly to the liquid; This should be done slowly and at as much of an angle as possible to prevent the formation of air-bubbles in the wall of the sac. Minute air-bubbles do not impair the efficiency of the sac but large ones are liable to burst during the subsequent pro- cess of sterilization. After the tube has been taken out and returned
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