8 SPECIFIC ROTATORY POWER . [BOOK I. 



rotatory power exerted by different bodies, care must be taken that the 

 nature of the light is the same. The light obtained by volatilizing sodium 

 compounds in a colourless gas flame affords an admirable source of light of 

 one uniform wave-length. 



The expression * specific rotatory power ' or * specific rotation ' is used 

 to designate the rotation (expressed in degrees) of the plane of polarized 

 light, produced by 1 gramme of substance dissolved in 1 cubic centimetre 

 of liquid when examined in a column 1 decimetre thick. 



Let a be the rotation observed, and p the weight in grammes of the 

 active substance contained in 1 cubic centimetre, and let I be the length 

 of the tube in decimetres, then if we designate by (a) D the specific rotation 

 for light having a wave-length corresponding to D, 



In this formula the sign + indicates that the substance is dextrogyrous, 

 the sign that it is Icevogyrous. In some cases the rotation is determined 

 for mean yellow light and not for Z>, and is expressed by (a) jf the value 

 of which is always somewhat different from that of (a) D . 



Various instruments have been devised and much employed in the 

 determination of rotation of the plane of polarization, especially in the 

 estimation of sugar, and are known by the terms Saccharimeters, Polari- 

 meters, and Polaristrobometers. One of the most convenient and most 

 widely employed is the saccharimeter of Soleil, which as modified by 

 Yentke and Hoppe-Seyler, enables the percentage of serum-albumin and 

 of glucose present in a liquid to be directly read from a scale attached 

 to the instrument. In this instrument the rotation is determined for 

 the mean yellow. 



The instrument of Soleil 1 has however been of late years sur- 

 passed by others, especially by those invented by Wild, Jellett, and 

 Laurent. A description of the latter instrument will alone be 

 given. 



Laurent's This instrument is shewn in Figs. 3 and 4. A Vis a 



Poiarimetre a Bun sen lamp. A (Fig. 3) is a small spoon of platinum 

 Penombres. gauze with the tip turned upwards, and in this is placed 

 a small quantity of common salt. The tip of the spoon is placed 

 in the outer flame, and when the salt is volatilized an extremely 

 brilliant sodium flame is produced. At B is a cell containing potas- 

 sium bichromate, which cuts off all but the yellow rays. To the 

 lever /is attached a double refracting prism which polarizes the light, 

 and at D (Fig. 4.) is a diaphragm of which one half is covered by a 

 plate of quartz. This serves to modify the light in a manner 

 explained in the account of the theory of the instrument. The 



1 For the description of the Soleil- Ventke Saccharimeter, and of Wild's Polari- 

 strobometer, the reader is referred to Hoppe-Seyler 'a Handbuch der physiologisch-u. 

 pathologisch-chemischen Analyse, and for a fuller description of these instruments, 

 as well as for a discussion of the whole subject of rotatory polarization, to Professor 

 Llandolt's recent work entitled Das optische Drehungsvenuogen organischer Snb- 

 stanzen und die practischen Anwendungen desselben. Braunschweig, Vieweg und Sohn, 

 1879, p. 237. 



