10 : PREPARATION AND MOUNTING 
solutions. There are some precautions needed for perfect 
success with this agent, for which the reader is referred to 
Frey’s “ Microscopic Technology.” 
Bichromate of potash, in solutions of similar strength to 
those of chromic acid, may be used in the same way, but is 
far slower in producing its effect, and therefore inferior in 
the opinion of many. Stricker, however, says, “that it has 
the great advantage that tissues saturated with it do not 
become friable, and that the time occupied by this agent, 
as well as by the preceding, may be much shortened by re- 
moving the preparation into alcohol for twenty-four hours.” 
It is always advisable to divide the substances to be har- 
dened into portions as small as convenient, since the larger 
often putrefy in the centre, though they harden at the sur- 
face. It is quite certain that many of the more delicate 
structures, such as the rods of the cochlea of the ear 
(Pritchard), those of the eyes of insects, &., are better 
prepared with this than by the preceding agent. One great 
element of success in these two processes is, that the volume 
of the solution should be very large in proportion to the 
size of the object; another, that the action should be 
commenced with a weak solution, and continued with a 
stronger. 
It sometimes happens that objects may be hardened too 
much by these solutions, though there is less risk by the 
bichromate of potash. In such cases Frey recommends that 
they be soaked in glycerine for a few days, and even that it 
be added to the solutions at first. He, with Deiters, Arnold, 
Schultze, and Kihne, claims for these solutions an effect of 
the most important kind, viz., that of “ preserving the finest 
textural relations, while exerting a somewhat macerating 
action on them, so that very delicate organizations, especially 
in nerve tissues, may be made visible which were previously 
hidden, or not visible in examination of the fresh tissue.” 
Hyperosmic acid and chloride of palladium are sometimes 
used for this purpose also. Their solutions may contain 
from one-fifth to one-tenth per cent. of distilled water. 
