cell-sap are supposed to prevent the precipitation of the protein by 
the tannin. When these acids are neutralised a protein-tannin 
precipitate is produced according to Prerrer. 
Prerrer thinks that the formation of the precipitate in Spirogyra 
must be explained otherwise than the precipitation of tannin by 
ammonium carbonate, because in Spirogyra filaments a precipitate 
occurs with ammonium-carbonate at greater dilution than in solutions 
of tannin. Af Kiercker') has erroneously considered this observation 
incorrect. I have indeed found it to be correct and I have also 
come to the conclusion that organic acids can entirely or partly 
prevent the precipitation of protein and gelatin by tannin. 
On the other hand, in order to explain his observations PFEFFER 
assumes various factors, without proving their existence, whilst he 
takes no account of other existing factors. In the first place PFEFFER 
ought to have considered whether the tannin in Spirogyra is really 
identical with gallnut-tannin. It is quite possible that the tannin in 
Spirogyra is a different chemical body from gallnut-tannin and behaves 
rather differently towards ammonium-carbonate. Then Prerrer has 
failed to demonstrate the presence of organie acids in the cell-sap. 
Also he has not proved the presence of protein in the precipitate 
and moreover he has not investigated whether the formation of the 
precipitate may be influenced by other substances. 
As to the first point, 1 have found that gallnut-tannin and Spirogyra- 
tannin in general behave similarly towards reagents and solvents. Also a 
solution of ammonium-carbonate must be more concentrated in order 
to produce in a solution of Spirogyra-tannin a precipitate than is 
necessary to produce it in the living cells of Spirogyra. The first 
point may therefore be left. 
It is otherwise with the presence of acids in the cell-sap. When 
Spirogyra is washed out and then disintegrated, the mass has a 
faint acid reaction to litmus paper but a solution of gallnut-tannin 
and of Spirogyra-tannin are likewise acid. A suitable microchemical 
method for demonstrating free acids in the cell-sap, does not appear 
te exist. No value can be attached to Loew and Boxorny’s?) method. 
They lay filaments of Spirogyra in a potassium iodide solution and 
seeing that no iodine is set free, they infer the absence of free acid 
in the cell-sap. The liberation of iodine by free acid cannot be explained 
chemically, for although dilute acids might set free hydriodie acid 
from potassium iodide, they cannot liberate iodine. 
ye pal ft 
*) O, Loew and TH. Bokorny, Ueber das Vorkommen von activem Albumin 
im Zellsaft und dessen Ausscheidung in Körnchen durch Basen. 1. c. 
