318 BATES AND PHELPS : TESTING OF SUGARS 



The constant, C, has been determined for a number of different 

 cases: (1) When the solution is poured back upon the filter after 

 all has run through; (2) when it is poured back after about one- 

 half hasrun through; (3) when it is not poured back at all. Ob- 

 servations were made by two methods: First, b}^ weighing the 

 solution at intervals during the filtration; second, by observing 

 the change in polarization. In the polarization method one tube 

 was filled, as a reference tube, with solution that had been covered 

 during filtration, since filtration could not be avoided. It is 

 shown, however, by weighing, that the evaporation in this case is 

 quite negligible. Other tubes were filled with solution which had 

 not been covered during filtration. The difference in polariza- 

 tion between these tubes and the reference tube is the change 

 due to evaporation. This was done under various atmospheric 

 conditions. 



The observations were made in a thermostated room where the 



amount of moisture present in the air, as well as the temperature, 



could be varied at will, the range of adjustment being (Ps — -Pa) 



= 4 mm. of Hg to (P., — Pa) = 41 mm. of Hg. In practice 



Ps — Pa rarely exceeds 22 mm. 



It was found that in the case where the solution was not poured 

 back upon the filter the change in polarization due to evaporation 

 is so small as to be quite negligible in ordinary sugar testing. 

 The change is represented by Q = 0.00017 (Ps - Pa) T. How- 

 ever, in case a portion of the solution is poured back, the change 

 due to evaporation is not negligible in ordinary testing but must 

 either be prevented, by covering the funnel, or corrected for by the 

 equation Q = 0.0006 {P, - Pa) T. 



CRYSTALLOGRAPHY. — The crystallographic and optic prop- 

 erties of magnesium and manganese pyrophosphates. Olaf 

 Andersen, Geophysical Laboratory. Communicated by 

 Fred. E. Wright. 



Crystals of magnesium and manganese pyrophosphates were 

 obtained by melting the substances^ and cooling the melts. 



1 The pyrophosphates were produced by heating magnesium (or manganese) 

 ammonium orthophosphate. The thermal examination of these and other sub- 

 stances is dealt with in a different paper. Mg2P207 melts at 1383°C., Mn2P207 

 at 1196°C. 



