242 SEMI-CENTENNIAL OF TORREY BOTANICAL CLUB 
represent with some accuracy the maximum concentration of the 
soluble substances present. Even here, however, there may be 
room for error. With the ordinary means at hand, pressure is 
not the easiest thing in the world to standardize and it is possible 
that identical samples might yield juices of slightly different con- 
centration if subjected to different pressures. The speaker is 
quite aware that such errors may have crept into his own work, but 
feels rather confident from somewhat empirical tests that have been 
made that the error cannot be an important one. 
Where, however, what may be termed total acidity is desired, 
a single pressure no matter how powerful opens the road to serious 
mistakes. By total acidity I mean the total acid content of a 
given weight of fresh or air-dry tissue. It is here that it is very 
difficult to tell in many published accounts how great have been 
the precautions not to leave a considerable percentage of acid in 
the rejected pulp. It is to be presumed that in most instances 
investigators were fully aware of this danger, but rarely is there 
any mention made of the procedure used to obviate the diffi- 
culty. 
In his own work the writer has found that what appears to be 
a very close approximation of the actual total acid content may 
be obtained by repeated pressure. After the first juice had been 
expressed and the press released the remaining pulp is collected 
and copiously moistened with water, which it greedily absorbs. 
This is then pressed once more and the process repeated until the 
final expressed water shows by titration a practically negligible 
amount of acid. 
All of the samples so obtained are mixed and made up toa 
definite volume, an aliquot portion of which is then titrated. In 
regard to the number of times this process is repeated each tissue 
will no doubt show its own peculiarities, In some very refractory 
tissues it might be an exceedingly difficult matter to satisfy oneself 
that the last trace of acid was extracted. In the tissues with 
which I have worked, notably the succulents, it has been found 
that the water which comes from the fourth pressing is nearly 
acid-free. Here again it is impossible to lay down a rule, for the 
type of press used might influence the result. Only by actual 
testing can we be sure that the acid is extracted. 
