SOME VEGETABLE PROTEIDS. 127 



shadow " polarimeter of the Laurent type,* constructed by J. 

 Dubose, of Paris, was employed to measure the rotary power of 

 the proteid solutions. This was a large instrument designed 

 for use as a saccharimeter and capable of holding tubes 400 

 and 500 millimeters long, with a 20-centimeter circle divided 

 to half-degrees, and a vernier with magnifying glass reading 

 to 0.05. Sodium light was alone used, as no other mono- 

 chromatic light could be found of sufficient intensity. The 

 exact position of the analyzer when the two halves of the field 

 became of the same shade could not be determined by a single 

 setting closer than 0.05 to 0.10. However, by taking a large 

 number of observations and averaging them, the error of the 

 mean could be reduced to less than 0.01. In general, from 

 twenty to forty settings were made and readings taken, the 

 position of uniform shade being approached alternately from 

 the right and the left hand. The temperature of the solution 

 in the tube was found by removing the cover and inserting a 

 thermometer immediately after the rotation had been measured. 

 Instead of depending on the result of one such measurement 

 of the rotation, the tube was always emptied and refilled with 

 a fresh portion of the solution and a new series of readings 

 were taken. This was repeated a number of tunes, and the 

 weighted mean of the results obtained was adopted as the best 

 value. 



The specific rotation was calculated by the formula 



, _, 



where a is the angle of rotation measured, I the length of the 

 tube in decimeters, and c the weight of ash- and water-free 



substance in 100 cubic centimeters. The factor -= in this 



I c 



investigation varied, according to the percentage of proteid 

 in the solution, from 50 to as large a value as 200. Thus 

 an error of 0.01 in measuring the angle of rotation, a, meant 

 an error of from 0.5 to 2.0 in the value of the specific 

 rotation, (a) D . 



* Dingl. Polyt. Journ., vol. ccxxiii, p. 608. 



