CANE SUGAE. 



The second normal weight, used in instruments of the Laurent pattern, 

 and in the old Duboscq instruments, is based on the angle through which the 

 plane of polarization of the D ray of sodium light is rotated by a plate of 

 quartz 1 mm. thick, cut at right angles to its optical axis ; 

 this angle, when the sodium light obtained by vaporizing 

 sodium chloride in a bun sen burner is filtered through potassium 

 bichromate, is 21-667, and this is the rotation produced by a 

 solution of 16-29 grms. of sugar in 100 true c.c., whence this 

 weight of sugar was adopted as a standard normal weight. This 

 normal weight has been much confused ; the early Duboscq 

 instrument used 16-35 grms. as a normal weight ; other instru- 

 ments were standardized for a normal weight of 16- 19 grms. 

 The confusion has in part arisen by determinations of the optical activity of 

 quartz under conditions other than those originally stipulated ; actually, the 

 rotation of quartz is immaterial, so long as it is known for what normal weight 

 and for which cubic centimetre the instrument has been constructed. 



The Compensator. The compensating arrangement consists of a 

 device whereby a variable thickness of active quartz may be interposed and a 

 rotation equal in magnitude and opposite in sign to that due to the active 

 material introduced, so that the reference point again appears ; the compensator 

 used in most sugar instruments is shown on an enlarged scale in Fig. 235 ; it 

 consists of a plate of levo-rotatory quartz c, and of two wedges of dextro-rotatory 

 quartz, a and I ; by means of a rack and pinion gear one of the wedges is capable of 

 being slid past the other, so that the combined thickness of the system is capable 

 of being varied ; on the moving wedge is fixed a scale graduated in single 

 degrees from 30 to + 105 ; on the fixed plate of quartz is mounted a vernier. 

 When the scale is at zero, the combined thickness of the dextro-rotatory 

 wedges a and b is equal to that of the levo-rotatory plate c, so that the effect 

 of the system is zero. By sliding the scale towards the 100 point a diminished 

 thickness of dextro-rotatory quartz is introduced, so that the effect of the system 

 is levo-rotatory, and at the 1 00 point exactly neutralizes the rotation produced by 

 the normal weight of sugar dissolved in 100 c.c., and observed in a 20 cm. tube. 

 Reading the Scale. The scale of these instruments is shown in 

 Fig. 236 ; it consists of two parts, the lower one fixed, and the other sliding, 



and moved simultaneously with the 1 



adjustment of the compensator. The ..It I. I.I. i J J I I. I. I J.I ,1 J J J-l I ', U 

 line at which the reading is taken 



1111111 



is that marked in the upper scale. FIG. 236. 



At the zero position this line is continuous with that line marked in the 

 lower scale. To take a reading the position of the zero line on the upper scale 

 is noted with respect to the lower scale. In Fig. 236 this is between 26 

 and 27. It is next observed what lines on the two scales are continuous ; in 

 this case the seventh, counting from zero. The full reading is then 26-7. 



434 



