1339 
I attempted to demonstrate free acid in the living cells of Spirogyra 
as follows. I placed Spirogyra in a solution of potassium iodide 
(0.1°/,) and of potassium iodate (0.025°/,), but no separation of iodine 
by free acid was indicated (5KI+-K1O,+6HCI— 6KCI+6I-+3H,0). 
On heating Spirogyra for some time in a 0.1°/, solution of citric 
acid, before placing it in the solution of potassium iodide and iodate 
a very faint blue colour in the starch and faint violet coloration of 
the nuciei was to be seen; the latter had taken up tannin from the 
cell-sap, for in the meantime the cells had perished. This result points 
to light absorption of citric acid and separation of iodine by this 
acid. The method seems to yield useful results and probably in the 
first experiment iodine would also have been liberated, in case 
Spirogyra contained free acid. 
It should be noted that Spirogyra is very sensitive to dilute solutions 
of organic acids. In a 0.1°/, solution of citric acid, tartaric acid, 
malic acid, quinie acid, it quickly dies. 
On these grounds it is very improbable that Spirogyra contains so 
much acid that protein and tannin should be able to appear together 
in soluble form in the cell-sap. The experiments which I am about 
to describe, also show that Prerrer has incorrectly interpreted his 
observations. 
Whilst with many reagents it is quite easy to demonstrate tannin 
in the cell-sap of Spirogyra because the cell-wall and protoplasm are 
permeable to these reagents, the most important tannin-reagents, 
namely, those which belong to the protein group cannot permeate. 
For this reason I heated Spirogyra in egg-white-, gelatin- or glue- 
solutions. 
On the death of the protoplasts the tannin passes through the 
protoplasmic layer and the cell-wall and a precipitate is formed 
outside the cell. If, instead of allowing the tannin to pass out, a 
little protein solution could be introduced into the cell-sap which 
contains the tannin and if we could investigate the result, this would goa 
long way in my opinion towards solving the problem of whether in 
the cell-sap protein exists in solution as well as tannin. Should the 
cell-sap remain clear, one might be able to assume that the cell-sap, 
was of such composition as to contain dissolved tannin and protein 
side by side. If, on the other hand, a small amount of protein- 
solution produced a precipitate, then this might be taken to exelude 
the simultaneous presence of the two substances. 
I will proceed to explain how I succeeded in introducing a protein- 
solution into the cell-sap, causing a precipitate which on closer 
investigation was found to be a compound of tannin and protein. 
