E. J. COHN, J. GROSS, AND O. C. JOHNSON 



151 



and the nitrogen content of the juices was determined after the 

 precipitate had been removed by centrifuging. Small amounts of 

 sodium chloride greatly decreased the volume of the precipitate and 

 also increased the solubility of the tuberin, though not to the same 

 extent. The effect of sodium chloride upon the solubihty of tuberin 

 and upon the apparent hydrogen potential of systems, containing it 

 is different not only in amount but in kind at different hydrogen 

 ion concentrations. The relation between this change and the iso- 

 electric point will be considered in another report. 



TABLE II. 



Effect of Sodium Chloride upon the Precipitation of Tuberin by Acid. 



NaCl added to 100 cc. of potato 

 juice + 5 cc. of N HCl. 



8.0 



The Solubility of Tuberin.- — The solubility of tuberin at different 

 hydrogen ion concentrations was estimated by determining the nitro- 

 gen content of potato juice to which sodium hydroxide or hydrochloric 

 acid had been added in different amounts and from which any result- 

 ing precipitate had been removed by filtering or centrifuging. The 

 results of four different experiments, in two of which the volumes 

 of the precipitate were also determined, are reported in Table I. 

 In each tuberin was least soluble in the juice of the potato at hydrogen 

 ion concentrations slightly greater than 10~^N, where precipitation 

 occurred. Precipitation also occurred at hydrogen ion concentrations 

 near 10~^n. The amount of these precipitates was very different, but 

 neither the addition of acid nor of alkali completely precipitated the 

 protein in potato juice. Indeed no more than 25 per cent of the total 

 nitrogen in the juice of the potato was ever found in the most 

 copious acid precipitate nor more than 8 per cent in the largest alkaHne 

 precipitate. A curve representing the solubility of tuberin in potato 

 juice thus passes through two minima and a maximum, the latter at 

 the reaction of potato juice (Fig. 2). 



