84 



CULTURE OF THE SUGAR BEET. 



Corrections for temperature are given by Champion and Pellet as fol- 

 lows : 



The method of determining the value of sacchariferous juices by means 

 of the specific gravity is that which will find widest application and give 

 the most satisfactory results in all hands outside the laboratory. The 

 second method mentioned, which is to some extent mechanical in its ap- 

 plication, and that may be used with limited laboratory appliances, 

 though more complicated than the first, is that depending upon the 

 rotary power of sugar solutions, or their power to influence the rotation 

 of the plane of polarization of a ray of light passing through the Nichols 

 prisms. It must be manipulated with care, however, and gives best re- 

 sults in practiced hands. The principal objection to the method is the 

 costliness of the apparatus, but its convenience and practical accuracy 

 cannot be questioned. Without entering further into a description of 

 the principles of the method, or of all of the forms of apparatus which 

 have been devised for the determination of sugar by this means, which 

 may be found in any of the leading works on chemistry, I will merely 

 call attention to the instrument lately devised by Laurent, of Paris, 

 which finds such favor with all the chemists and manufacturers who 

 have frequent occasion to estimate sugar in solutions, and who are dis- 

 carding other polariscopes on account of its many advantages. 



Laurent's instrument is a modification of that of Soleil. The differ- 

 ence consists in the introduction between the polarizer and analyzer of 

 a plate of gypsum or selenite of such thickness as to give to the ray of 

 light passing through the instrument a yellow tint identical with that 

 of the line D of the sodium spectrum. The plate covers half the field of 

 vision, so that the other half of the field, when white light is employed, 

 will have the color complementary to yellow or violet blue. The diffi- 

 culty of differences of appreciation of shades of color by observers is 

 obviated by using the monochromatic light produced by introduction of 

 a sodium salt into the colorless flame of the Bunsen burner. The slight 

 blue shade of the Bunsen flame, which is decomposed by the spectro- 

 scope into violet and green, is disposed of by causing the light from the 

 flame to pass through a solution of bichromate of potassium, which has 

 the well-known property of absorbing the green, blue, and violet rays. 

 The Selenite plate takes the place of the quartz prisms of. the Soleil in- 

 strument, and the analyzer is connected with a metallic circular disc, 

 which is arranged perpendicular to the axis of the instrument. The 



