202 A. B. MACALLUM. 
neglected in the making of permanent preparations. In order 
to prevent contamination of the sections with iron compounds 
in the demonstration, the solutions of potassic ferrocyanide 
were, on all the occasions used, not more than a week old, 
although I found that those of longer standing, up to the end 
of two months or so, when filtered carefully, gave preparations 
which were free from any objectionable characters. The 
strength employed was 1°5 per cent., and a volume of this 
was mixed with an equal volume of hydrochloric acid of 0°5 per 
cent. strength, when the mixture was required. 
The sections, after removal from acid alcohol, were first 
washed in pure alcohol, then in distilled water, after which 
they were placed in the acid ferrocyanide mixture for not more 
than five minutes. Again washed carefully in distilled water, 
they were either dehydrated in alcohol, cleared in oil of cedar, 
and mounted in benzole balsam, or, before they were put 
through this course, stained with either safranin or eosin. 
The staining reagents were of 1 per cent. strength in 30 
per cent. alcohol, and the time allowed for the action of the 
eosin was three minutes, while that for the action of safranin 
was half an hour. The excess of the stain in either case was 
removed with alcohol. The advantages given by the use of 
these stains I have explained in the description of the con- 
stituents of the nucleus. Very frequently I have found that 
a preparation which illustrated, in a remarkable way, some 
point in the distribution of iron in the cell, became useless 
through a complete fading out of the blue. The causes of 
this result are two: exposure of the preparation to the light 
for a time, and the use of inferior oil of cedar, that is, impure 
through the presence of minute quantities of water and other 
matters. I found that when I used old oil of cedar to clear up 
the sections and to remove all traces of alcohol, the prepara- 
tions would keep their beauty unimpaired, if placed away from 
the light in the slide box. In some way the preservation of the 
blue colour depends on leaving a trace of the oil used in 
clearing-up upon the section when the balsam is added, but 
in this quantity allowed to remain there must be no alcohol or 
