ESTIMATION OF CARBOHYDRATES. 



The methods of estimating carbohydrates depend ultimately on 

 the methods of estimating glucose. Though at first sight the estima- 

 tion of glucose may appear as a comparatively easy task, yet on 

 examination of the literature few subjects seem to have been more 

 worked at than this simple problem. Over thirty methods have 

 been devised by the most distinguished chemists and new ones are 

 continually being described and advocated. 



Three of the properties of glucose (and other carbohydrates) are 

 most usually made use of for its estimation : 



A. Its optical activity, by means of the polarimeter. 



B. Its aldehyde character, by the reduction of metallic salts,, 

 especially copper. 



C. Its fermentation, by yeast. 



Each of these methods has its own particular advantages, which 

 depend mainly upon its convenience, ease of manipulation, rapidity of 

 completion, and desired accuracy. 



A. ESTIMATION BY MEANS OF THE POLARIMETER. 

 i. The Construction of a Polarimeter. 



In an ordinary ray of light the vibrations of the waves take place 

 in all planes perpendicular to the direction of its propagation. If 

 such a ray of light be passed through a crystal of Iceland- or calc-spar 

 and an object be observed through the crystal, two images will be seen. 

 The ray of light has been split into two rays, one of which has been 

 more refracted than the other. The more refracted, or ordinary, ray 

 travels through the crystal just as it would travel through glass and 

 obeys the laws of refraction. The less refracted, or extraordinary, ray 

 does not obey the ordinary laws of refraction, and it shows a movable 

 image when the crystal is rotated. Both of these rays in their passage 

 through the crystal have been polarised in two directions at right 



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