COLLODIUM SACS. 497 



be used in making- sacs. This is notably true of celloidin solutions. 

 The film of collodium 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 purposes. 



FIG. 70. The rolling of collodium sacs. The beaker contains three sacs. 



The collodium is placed in a small glass cylinder, about 4 cm. in 

 diameter and 9 cm. hig-h. This is inclined in the manner indicated in 

 Fig-. 70. The clean, dry glass tube is inserted into the collodium 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 collodium and rolled in the air. 

 When the tube is returned to the liquid another coat of collodium is 

 deposited. By repeating- this several times a g-ood layer of collodium 

 is deposited on the glass. It may happen that while the collodium 

 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 

 difficulty can be overcome by completely withdrawing- the tube out of 

 the collodium, 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 

 to the collodium three times, it will usually have formed upon its 

 surface a sufficiently thick layer of collodium. 



The tube is then rotated slowly in the air to allow the collodium 

 to partially set. By proper rotation the liquid can be prevented from 



