1877.] The Study of Zodlogy in Germany. 395 
drops of strong carbolic acid may be added to the water to pre- 
vent the development of bacteria, ete., which would quickly ruin 
the preparations. Coagulated gum renders the majority of or- 
gans of a pleasant consistency for cutting. 
Instead of gum, paraffine may be made to permeate the tissues, 
in the way already described in detail in the article on the 
sledge microtome, in the April NATURALIST. 
All acids produce in albumen chemical changes, which, with- 
out withdrawing the water, cause coagulation. There are some 
which are admirably suited for hardening agents. Foremost 
among these is chromic acid, first introduced by Hannover, in 
1841, from motives of economy. It is employed in solutions of 
two fifth parts for one thousand parts water. Very large quan- 
tities must be used,— weak solutions at first to be gradually 
replaced by stronger and stronger ones. If its action is kept 
up too long the objects become brittle and are then worthless, 
for every section crumbles to pieces as soon as made. Chromic 
acid is particularly useful in studying nervous tissues, organs of 
sense, and other unusually delicate tissues. Its action is very 
slow: thus the spinal cord of a large dog or a man requires at 
least six weeks or two months. Chromic acid is also admirable 
for preparing very young and frail embryos or eggs. There 
are many other agents which are sometimes used for hardening, 
but it is not deemed appropriate to eriumerate here any but 
the two principal and most useful ones, alcohol and chromic 
acid. 
After the proper degree of hardness has been produced, if the 
piece to be cut is large enough, it may be held in one hand and 
cut with the other without more ado. When, however, we have 
to deal with something too small and delicate to be held in the 
hand, it is necessary to have recourse to some method of im- 
bedding. Paraffine will usually be found the most convenient 
substance for this purpose, especially when mixed with one tenth 
of its weight of the best hog’s lard. The most ahisfactory proc- 
ess of imbedding in paraffine we have elsewhere described.! 
On some accounts transparent soap is to be highly recom- 
mended. The best quality, containing no glycerine, must be 
chosen, then shaved into small bits, and warmed with half its vol- 
ume of alcohol (as compared with it before it was cut up) 
until it is entirely dissolved ; the specimen to be imbedded is 
then suspended in the warm mass by a fine thread and left for 
1 April NATURALIST, 1877, page 208. 
