IRON COMPOUNDS IN ANIMAL AND VEGETABLE CELLS. 181 
maximum reaction in about ten days, while the latter mani- 
fested a reaction of moderate intensity at the end of that time, 
which, with a longer stay in the warm oven, did not become 
more marked. In the case of vegetable cells the reactions 
were more quickly obtained and the differences in intensity 
greater. This is illustrated in figs. 14,15, and 16, representing 
preparations of cells of the ovary of Erythronium ameri- 
canum, in which the reagents used had been made from dilute 
solutions of ammonia (of 0°96 sp. gr.). Fig. 15 indicates the 
depth of the reaction with ammonium hydrogen sulphide at 
the end of twenty-four hours, the intensity attaining in 
another cell in ninety hours the degree represented in fig. 16, 
while in fig. 14 is shown how far the reaction had progressed 
with diammonium sulphide in forty-eight hours. In the latter 
case the reaction did not become more marked even on the 
eighth day. Similar results were obtained in all the experi- 
ments of this character, demonstrating that ammonium 
hydrogen sulphide is more effective in liberating iron from 
organic combinations than is the diammonium compound. 
In the earlier stages of the investigation the reagent was 
made from strong solutions of ammonia of sp. gr. 0°88; but 
when thus prepared it deteriorates rapidly and becomes yellow 
from the formation of polysulphides. Spoiled or unsuc- 
cessful preparations were consequently frequently obtained. 
Sometimes, also, difficulties were experienced in determining 
whether, in the preparation of the reagent, the saturation of 
the strong ammonia with sulphuretted hydrogen was complete. 
For this reason, and also because dilute solutions of ammonium 
hydrogen sulphide are less unpleasant in every way, I began 
to use the latter, and found that it gives results not less decided 
than those obtained with the stronger solutions. The dilute 
solutions offer other advantages, for when made from pure 
ammonia of 0°96 sp. gr., they retain their potency for three 
weeks or longer, especially if kept in a bottle with a well-fitted 
glass stopper, and in a cool place. The smaller the amount 
of air in the bottle and the less frequently the stopper is 
removed, the longer does the reagent retain its strength, 
