308 BOTANICAL GAZETTE [ocTOBER 
are scarcely at all colored, the mixtures being somewhat of a purplish color. 
These observations seem to accord with the experiments of Rouvrer (13), 
who found that the different classes of starch took up varying amounts of 
iodin, and are contrary to the statement by Stocks (g) that the shade of 
color varies with the amount of iodin but not with the different starches. 
In other words, the shade and intensity of color not only vary according 
to the strength of iodin solution, but also according to the kind of starch 
used. 
It is well known that if a starch solution be treated with iodin in the 
cold, a blue coloration will be the result, and that if this solution be then 
heated the blue color will disappear, but will return on cooling the solution, 
though less intensely than before. This peculiar behavior of starch and 
iodin has never been satisfactorily explained. NAGELI and SCHWENDENER 
(14) consider that the loss of color on the application of heat is due to the 
production of hydriodic acid; but this does not explain the reappearance 
of the color on cooling; and so far as I am aware no one has ever demon- 
strated the presence of thisacid. If this acid were present, then according to 
Myutus (7) and MEINEKE (8), the blue color would remain on heating 
solution, since they claim, as already pointed out, that the blue color 1s due 
to the presence of this acid. It is thus apparent that there is considerab 
contradiction in the arguments that have been presented on this subject. 
The following experiments tend to explain this phenomenon. If we 
take 0.5%" of potato starch and mix it with so°° of water and heat the 
mixture in an Erlenmeyer flask of about roo°¢ capacity on a water one % 
a temperature of about 80° C. for one hour, and then add 5° of iodin 
solution (iodin 0.1 per cent. and potassium iodid 0.5 per cent.) and place 
a piece of ordinary filter paper, which usually contains starch, over ert 
of the flask, and then continue the heating until the solution is decolo: 
which takes place at about 80° C., the filter paper will be colored blue, 
showing that some of the iodin has been volatilized. Furie . 
cooling the solution, as already pointed out, the blue color begins to pe 
at about 65° C. and increases in intensity until at about 40° C. it sen a 
the maximum; though it is less intense than the color produced Ww. ee 
iodin solution is added to a starch solution cooled to this temper? ; 
These results may be interpreted as showing that iodin forms 4 ee ae 
with starch which is dissociated on the application of heat; ai fe 
temperature between 60 and 80° C., part of the freed iodin 1s ee ngth a 
such, and the amount remaining in solution depends upon the } ge 
time the heat is applied. Also the loss of iodin in this way accoum 
the decrease in color of the starch-iodin solution on cooling. 
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