EXPERIMENTAL METHODS. 11 



The usual method of acid extraction was this: A sufficiently large sample, 

 say of about 50 grams of tissue of healthy joints, was pressed in an ordinary 

 large-sized meat-press. From such a sample there were obtained usually at 

 least 10 c.c. of what is termed in the succeeding pages "pure juice." The 

 writer expressly avoids the term "cell-sap," which means little or nothing and 

 is apt to give a very erroneous impression. From young and very turgid 

 tissue the amount of juice so obtained was sometimes as much as 20 or even 

 25 c.c., and from old flaccid tissue as low as 5 c.c. With the latter material, 

 indeed, it was sometimes necessary to use a considerably larger sample in order 

 to obtain enough juice to give a satisfactory result, for in all but a very few 

 cases the titrations of pure juice represent at least 10 c.c. This juice as it 

 came from the plant was of course discolored with chlorophyll and finely 

 comminuted tissue and was strained or filtered before titration. When 

 diluted with several times its bulk of water it made a very acceptable solution 

 for titration purposes, and its slightly greenish color, as soon as the eye 

 became accustomed to discounting its presence, did not interfere seriously with 

 a satisfactorily sharp reading of the end point. 



This sample of the tissue, already once pressed, was wetted and pressed 

 again and this process repeated a second and third tune. Trial showed that 

 the amount of acid obtained from a fourth pressing was negligible. The tissue 

 that remained was washed into a flask and shaken vigorously with water. 

 All extracts obtained by the second and subsequent pressings and the water 

 in which the squeezed tissue had been shaken were mixed together and strained 

 and used later in determining what has here been called "total acidity. 



In the actual titration as much of the pure juice was taken as could be 

 accurately measured, and in almost all cases it was seen that this should be at 

 least 10 c.c. Phenolphthalein was then added and the alkali run in from the 

 burette until the end-point was satisfactorily determined. In doubtful cases it 

 was sometimes purposely over-run and titrated back with standard acid. The 

 solution most generally employed for titration was an N/ 10 KOH, though in 

 some instances, where little acid was expected, an N/20 solution was employed. 

 As given in the tables, all results have been reduced to the decinormal equiv- 

 alent. The acid used for titrating back was an appropriate strength of HC1. 



Any remaining juice and the washings of all the apparatus which had con- 

 tained it were added to the secondary extractions, already spoken of, and the 

 whole made up to some definite quantity, usually 250 c.c. An aliquot portion 

 of this was titrated, as with the pure juice, at least two titrations being made 

 in every case. All of these processes can be carried on with considerable speed, 

 and while it is possible that this simple method for determining the acidity 

 may not be susceptible of as great accuracy as a more elaborate one, it has two 

 distinct advantages: It is possible to make many more observations in a 

 given time, and the juices are titrated in a practically unaltered condition. 

 No question of changes that might have taken place in the processes of extrac- 

 tion can come in. It is, indeed, true that not every plant, certainly very few of 

 the cacti, yield their acid so easily and in a form so little contaminated as does 

 Opuntia versicolor, and, as has previously been stated, it was for this reason that 

 this species was so largely employed. The only trouble that arose was the 

 occasional presence of the mucilaginous substances which are so charac- 

 teristic of the cacti, but these rarely occurred in considerable quantity. When 



