26 THE EXPERIMENTAL INDUCTION OF DEPENDENT NUTRITION. 



this way than when the juice was evaporated with pumice stone or with 

 asbestos, as described in Bulletin No. 107 (Revised), Bureau of Chemistry, 

 U. S. Department of Agriculture, p. 77. The organic matter was then 

 burned off and the amount of inorganic solids present in 25 c.c. of the 

 original juice obtained by igniting the residue at low redness to constant 

 weight. 



DETERMINATION OF ACIDITY. 



With the exception of opuntias, the juice from a sample was readily 

 obtained by first passing the soft pulp of the plant through a sausage-mill, 

 after which it was wrapped in a strong cloth and the juice pressed out in 

 a Biichner press. To determine the acidity, 25 c.c. of the juice thus 

 obtained were diluted with about 300 c.c. of recently -boiled water and then 

 titrated with standard sodium-hydroxide solution, using phenolphthalein 

 as an indicator, as directed in the official methods of the Association of Offi- 

 cial Agricultural Chemists. (Bur. Chem., U. S. Dept. Agr., Bulletin No. 

 107 (Revised), p. 79.) 



In some cases the end-point was difficult to determine on account of the 

 dark color of the juice. When this was the case the juice was diluted to 

 a still larger volume and a check made of the direct titration by adding 

 an excess of the standard sodium-hydroxide solution and then standard 

 hydrochloric-acid solution until the red color had disappeared. From the 

 amount of sodium hydroxide required, as shown by the mean of five titra- 

 tions, the acidity of the juice was expressed in terms of sulphuric acid. 



On account of the slimy nature of the juice of the opuntias it was found 

 impossible to separate the sap from the pulp, even when subjected to the 

 greatest pressure attainable with the Biichner press. A somewhat different 

 method of procedure was therefore necessarily adopted. The pulp was mac- 

 erated as thoroughly as possible, first in a sausage-mill and then in a 

 coffee-mill. The volume of the pulp thus obtained was measured in a 

 graduated cylinder, the true volume being found by subtracting from the 

 apparent volume the volume of water required to fill the interstices. More 

 water was added until a dilution of 1 part of pulp to 2 parts of water was 

 obtained. The mixture was then stirred thoroughly and finally filtered 

 through a coarse cloth to remove any unground pulp which might be pres- 

 ent. To determine the acidity 75 c.c. of the filtrate was diluted with boiled 

 water and titrated with standard sodium-hydroxide solution, as already 

 described. No correction was made for mark in this determination, as the 

 error thus involved is not great and lies within the limit of experimental 

 error to be expected on account of the exceedingly slimy nature of the 

 juice. 



