10 PREPARATION AND MOUNTING 



solutions. There are some precautions needed for peifect 

 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. Strieker, 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, &c., aro 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 Kiihne, claims for these solutions an effect ot 

 the most important kind, viz., that of "preserving the finest 

 textnral 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. 



