184 A. B. MACALLUM. 
proper proportions, of glycerine and ammonium hydrogen 
sulphide, for in such the reagent is more rapidly converted 
into the non-active form than whenit is kept separate. Appa- 
rently also in “stock” mixtures the polysulphides are very 
rapidly formed, the fluids becoming deep yellow in twenty-four 
hours or less, although the sulphide used may be nearly colour- 
less. In summer the change of colour is rapid. That it is 
due in part at least to the formation of polysulphides, appears 
to follow from the fact that drops of the mixture, when 
allowed to remain uncovered on the slide for a few minutes, 
quickly become milky in appearance from the precipitation of 
free sulphur. The mixtures retain a part of their strength 
during the first two or three days, after which they become 
useless. 
The tissues which were teased out for treatment were always 
hardened in alcohol wholly free from iron in solution. Latterly 
I have employed for this purpose redistilled methylated spirit. 
I have not used in this connection material fixed with any of 
the mineral hardening reagents, since the latter frequently 
contain iron, the presence of which in dying cells and tissues 
might be held to contribute, under the influence of the harden- 
ing reagent, to the formation of firm organic compounds of 
iron. Some mineral reagents, moreover—as, for example, 
corrosive sublimate and osmic acid—are difficult to remove 
from the tissues upon which they have been allowed to act, 
and their presence in preparations treated with ammonium 
sulphide, which forms sulphides with these metals, gives 
appearances obscuring, in a greater or less degree, the occur- 
rence of iron compounds. 
To facilitate the teasing-out I frequently used sections made 
with a clean steel knife! covered with absolute alcohol, the 
cells of such sections readily separating, and yielding sometimes 
a number of free nuclei. In order to determine whether iron 
in an inorganic or albuminate form is present, and to what 
1 Jn my earlier paper (loc. cit.) I pointed out that the knife so used gives 
no iron to the preparation. All my observations for the last two and a half 
years have in no way called in question the correctness of this contention. 
