CHEMICAL BASIS OF THE ANIMAL BODY. 103 



amounts 1 . The experimental difficulties of detecting traces of sugar in 

 this excretion are considerable. There is no dextrose normally in bile. 



The probability that it is as dextrose that the carbohydrates are 

 finally absorbed from the alimentary canal has already been referred to 

 (p. 58). This corresponds with the fact that dextrose is the most 

 readily assimilable sugar, as is known from comparative injections of 

 the various sugars into the blood vessels and observations on their 

 subsequent appearance in the urine. 



When pure, dextrose is colourless and crystallises from its aqueous 

 solution in six-sided tables or prisms, often agglomerated into warty 

 lumps. The crystals will dissolve in their own weight of cold water, 

 requiring however some time for the process ; they are very readily 

 soluble in hot water. Dextrose is somewhat sparingly soluble in cold 

 ethyl-alcohol, more soluble in warm ; slowly soluble, but in considerable 

 quantity, in methyl-alcohol and insoluble in ether. 



It may be prepared by concentrating diabetic urine until it yields 

 crystals of dextrose ; these are then purified by recrystallisation from 

 methyl-alcohol. It may also be conveniently prepared by the action of 

 hydrochloric acid on cane-sugar dissolved in alcohol 2 . A freshly pre- 

 pared cold aqueous solution of dextrose possesses a specific rotatory 

 power for monochromatic yellow light of (a) D = +100. This rapidly 

 falls, especially on warming, until it may be taken as (a) D = + 52 < 5 for 

 solutions which do not contain more than 10 p.c. of the sugar. The 

 rotation is however dependent on the concentration of the solution 

 being least with very dilute solutions. 



The specific rotatory power of a substance is the amount, measured in degrees, 

 by which the plane of polarised light is rotated by a solution which contains 1 gram 

 of the substance in each 1 c.c. when examined in a layer 1 dcm. in thickness. 

 Since the amount of rotation produced in any given case is directly proportional to 

 the specific rotatory power, also to the weight of substance in solution and the 

 thickness of the fluid layer in which it is examined we have a=(a)xpxl or 



(a) = - , where (a) is the specific rotatory power, p is the weight in grams of the 



substance in 1 c.c. of the solution, I is the thickness in decimetres of the fluid layer 

 and a is the observed rotation. This equation provides a means of estimating sugars 

 quantitatively by measuring the rotation produced by a solution of unknown con- 

 centration in a layer of known thickness, the specific rotatory power being known 3 . 

 The instruments employed for measuring the amount of rotation produced by an 

 optically active substance are known generically as Polarimeters. In one class of 

 these instruments the source of light used is a brightly luminous sodium- flame, the 



1 For literature and results see Neubauer u. Vogel, Analyse des Harris (Ed. ix. 

 1890), S. 41. 



2 Soxhlet, Jn. f. prakt. Chem. (N.F.) Bd. xxi. (1880), S. 227. 



3 For details of the instruments and methods see Landolt, Das optische Drehungs- 

 vermogen organ. Substanzen. Hoppe-Seyler, Physiol. path. chem. Anal 1883, S. 24. 

 Miller's Chem. (Ed. by Armstrong and Groves), Pt. m. 1880, p. 569 et seq. 



