486 



CHAPTER XXIV 



Polarimeter Tubes. — The older form of polarimeter tube is shown in Fig. 

 316. It consists of a glass or metal tube with the ends ground exactly flush 

 and parallel. On either end a screw thread is cut. To fiU the tube, a glass 

 disc is placed on one end and secured b}^ the cap. The tube is filled in a 

 vertical position and the second glass disc slid over the end and the emergent 



Fig. 316 



meniscus, avoiding the fonnation of an air bubble. The disc is then secured 

 in position by a second cap. 



A second form of tube, Fig. 317, uses sprung metal caps for securing the 

 glass discs in place. 



The latest form of tube. Fig. 318, has an enlarged end into which an air 



Fig. 317 



bubble may be directed, outside of the field of vision. This form is very 

 convenient since, when making a series of observations, the tubes may all 

 be placed in a row with the enlarged ends together ; if the tubes be systemati- 

 cally reversed when read, the observer knows the one last read in case of 

 interruption. 



Fig. 318 



Another form, Fig. 319, eliminates the annoyance of the air bubble by 

 means of a cavity blown in the glass. It also affords means for the identifica- 

 tion of a particular tube in a series. This tube rests on shoulders and not 

 on the caps, a method due to the U.S. Bureau of Standards^'. 



The Laurent instruments are supplied with bayonet-fastening spring caps. 





Fig. 319 



The continuous tube of Pellet^** is a great time-saving device. One 

 method of using the tube is shown in Fig. 320. The material under examina- 

 tion is poured into the reservoir a, whence it flows through the tube displacing 

 material already contained therein. By mounting a T-syphon, b, as in- 

 dicated at the delivery end, the flow automatically stops when the level c is 

 reached. This appliance is most useful when many consecutive readings on 

 materials of about the same density have to be made. In fitting up the 



