ADVANCED PHYSIOLOGICAL CHEMISTRY 423 



placed in the tube the two halves of the field will become of different 

 tints, i.e. rotation of the plane of polarised light has occurred. In order 

 to measure the amount of this rotation, we must move the screw or 

 pointer connected with the compensator or analyser until the uniform 

 tint is again obtained. The amount of 'compensation' necessary is 

 read off on the scale and, if the holder be not 188-6 mm. long, the 

 necessary calculation is made in order to ascertain the strength of the 

 solution (for formula see below). 



To . estimate the percentage of sugar in urine the chief precautions 

 are, (1) to see that it is perfectly clear, and (2) to see that it contains 

 no proteid. 



In order to obtain a specific or comparative number (i.e. a result 

 always obtained under the same conditions) it is necessary to adopt a 

 standard. This consists of the rotation, in degrees of a circle, produced 

 by 1 gr. of the substance dissolved in 1 c.cm. of fluid and contained 

 in a tube 1 dcm. long. This is called the specific rotatory power and is 

 represented by (a)D. 1 It is determined by the following formula: 



(a)D= ,, 

 pxl 



where a = the observed rotation, 



I = the length, in decimeters, of the tube in which the solution is 



placed, . 

 j? = the weight in grammes of the substance contained in 1 c.c. 



solvent. 



The rotation produced by a substance depends upon its concentration 

 in a solution ; if, therefore, the index (a)D of any substance be known, 

 and its rotation be ascertained, its percentage P in any fluid can be 

 ascertained by the formula. 



100ft 

 "~d 



where s = (a) D. 



For rapidly and accurately determining the percentage of sugar in 

 any fluid (e.g. urine) the polarimeter and especially that form of it 

 in which the scale reads percentages of sugar is a very valuable 

 instrument. It is much used for this purpose in the continental clinics. 

 The Specific Rotatory Power of certain of the sugars is as follows : 

 Monosaccharides : Dextrose : + 52 '6. 

 Galactose : + 83. 



Laevulose : rotates a variable distance to left. 



Disaccharides. The (a)D of these carbohydrates changes when they 

 1 The 'D ' indicates that sodium light is used, 



