166 



that for equal concentrations, all sugars have about the same index of re- 

 fraction. Main 1 published tables of water content from refractive index 

 in 3907, and called attention to the accuracy of this method, as compared 

 with the true dry substance. Since that time the literature has been full 

 of articles on this method of determining the dry substance. 



All authors, with but few exceptions, claim much for this method as a 

 quick one and yielding comparable results. They all agree that the re- 

 sults so obtained are nearer the true dry substance than by obtaining the 

 dry substance from the specific gravity. The substances dissolved along 

 with the sugar seemingly have a closer refractive index to sugar than 

 specihc gravity. 



Working on syrups of various origins, I obtained the following aver- 

 age figures.'- The method for true dry substance was loss of weight in 

 vacuum at 70° C. The table of Prinsen Geerligs, and also his corrections 

 for temperature were used. These are given later in this paper. 



IN CASE OF MAPLE SYRUP. 



Thirteen samples were examined. In only one case was the refracto- 

 meter dry substance higher than the true, and in all others the true dry 

 substance was higher. This difference ranged from 0.20% to 1.34% with 

 an average of 0.50%. 



WITH CANE SYRUP. 



Ten samples were examined. In three cases the refractometer dry 

 substance was higher than the true by 0.16%, 0.34%, 0.62%. The other 

 cases range from 0.24% to 0.93%, or an average difference on the whole 

 of 0.29%. 



HONEYS. 



Twenty-four samples were examined. In 2 cases the refractometer 

 dry substance was higher than the true by 0.21% and 0.91%. In all the 

 rest it was lower by from 1.15% to 2.52%, with an average of 1.45%. 

 This is the greatest difference noted. One of three causes or all may ac- 

 count for this large difference. (1) The actual dry substance may not be 

 right, viz., this product may not give up all of its water at 70° in vacuum, 

 or, (2) the dextrin of the honey may change the refractive index of the 



a Inter. Sugar J. (1907), 9, 481. 



2 Note. See Jour. Amer. Chem. Soc. (1908), SO, 1443, for a previous paper on this subject 

 by the author. 



