27 
supposed to prevent the precipitation of the protein by 
the tannin. When these acids are neutralised a protein- 
tannin precipitate is produced according to Pfeffer. 
Pfeffer 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 Klercker :) 
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 con- 
sidered whether the tannin in Spirogyra 1s 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 differentiy towards ammonium- 
carbonate. Then Pfeffer has failed to demonstrate the 
presence of organic 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. 
Às to the first point, I 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. 
rep. 374 
