IOWA ACADEMY OP SCIENCES. 
149 
superior to alcohol is the verdict of everyone who has thor- 
oughly tested it. It is true that it was severely criticized soon 
after its introduction into America, by certain workers who 
failed to secure permanent preservation with it. In all such 
cases of failure the solutions employed were very weak ones. 
A proper strength of solution is a very important detail. A 
solution of 4 per cent strength — that is, one containing 
Commercial formaldehyde 4 volumes, 
Water... 96 volumes, 
is perfectly safe for most objects. Of course, stronger solutions 
are required for special cases, and slightly weaker ones for 
others. 
Certain precautions in the use of this reagent require notice 
here: 
First . — The gas is quite volatile, and the containing jar must 
be kept tightly sealed. If it be impossible to entirely prevent 
evaporation, changing the solution occasionally will answer per- 
fectly well. 
Second . — The solution being an aqueous one it is liable to 
freeze. This probably appears, at first, a very serious matter, 
because we are so used to alcohol as a preservative, and this 
does not become frozen. 
Third . — The gas is irritating to the eyes, nose and throat. 
The effect, however, is merely temporary. Prolonged washing 
in water before a dissection is to be made will remove much of 
the reagent and reduce the annoyance to a minimum. Alcohol 
of 70 per cent strength appears to extract formaldehyde more 
rapidly than does water, but it is not always practicable to 
use it. 
II. THE USE OF FORMALDEHYDE IN FIXING AGENTS. 
In cellular biology the choice of a fixing agent means a great 
deal, All the conceptions which we build up about the cell 
appear to rest primarily upon the character of the reagent 
which was used in killing it. While we constantly seek to keep 
in our preparation the features of the living cell, how far short 
of the ideal we often fall every histologist knows. It is prob- 
able that certain recent investigations in cell structure will have 
to be gone over again because of too blind a faith in the fixing 
agents which were used. 
Formaldehyde alone is not suitable for general cytological 
work. It has a tendency to produce a vacuation in protoplasm 
