488 



CHAPTER XXIV 



made of glass, and in others it is engraved on the quartz wedge. The appear- 

 ance of the scale is as in Fig. 323, where the reading is 26.7. 



Control of the Scale. Quartz plates, the exact value of which has been 

 determined in sugar degrees, may be obtained from makers. These plates 

 are standardized at 20 C. in Europe, and as they are equally affected by 

 variation of temperature with the quartz wedge, they will serve at any tern- 



JTn i i { i i i > hi i I i [ i i Tl 



FIG. 322 



FIG. 323 



perature to control the scale of a polarimeter of this type ; but, if used to 

 control the scale of a polarimeter compensating by rotation of the analyser, 

 the correction for temperature must be applied. 



A control observation tube by Schmidt and Haensch is shown in Fig. 



'' 324 ; it consists of an outer tube, T, in which is moved by means of a rack- 



. and-pinion gear the tube/, fitting closely into T, exit of liquid between T 



and / being prevented by the washer e : the tube / is closed by a glass disc 



at c. The solution to be used for testing is poured into the funnel a, whence 



/%_>\ 



v-vAv,^-v-'J!v*<^ vbK^^^ I 



==3 1 HJ 



FIG. 324 



it fills the tube T. The distance between d and e is regulated by the rack- 

 and-pinion gear, the exact distance and also the length of the column of 

 liquid being read off a scale carrying a vernier ; the tube is conveniently 

 filled with a solution of the normal weight of sugar in 100 cc. ; with a column 

 of liquid 20 cm. long a reading of 100 should be obtained, and other readings 

 should be proportional to the length of the column of liquid. A very rapid 

 control over the scale can thus be obtained. Of course, the scale can also be 

 tested by polarizing different weights of pure sugar in a tube of constant 

 length, but this, compared with the adjustable control tube, is a laborious 

 operation. 



