Appendix. 1 5 1 



strength from 50 to 95%, is also frequently used as a fixing 

 and hardening agent. To these might be added osmic 

 acid and cupric sulphate. 



The length of time that objects should be allowed to 

 remain in the fixing fluids varies according to the fluid used 

 and the tissue or animal being treated. As a general rule, 

 a large quantity of fluid, two or three times the bulk of the 

 object, should be used, and this renewed as soon as it 

 becomes cloudy. 



F. 



PRESERVATION. 



OBJECTS that have been sufficiently fixed or hardened are 

 thoroughly washed in water, excepting such as have been 

 treated with picro-sulphuric acid or alcohol, and placed in 

 70% alcohol. An abundance of alcohol should always be 

 used. Picro-sulphuric-* preparations should be repeatedly 

 washed in alcohol of 70% before being finally placed in the 

 cabinet. 



G. 



METHODS OF STAINING. 



ORGANISMS that when living are brightly colored are gen- 

 erally bleached by the action of the fixing or preserving 

 media, and tissues which are really very dissimilar in struc- 

 ture not infrequently appear to be quite similar. To reveal 

 these masked or hidden structures, microscopists have made 

 use of certain stains. 



Stains are either diffuse or selective. The former dye the 

 tissues throughout, while the latter stain only certain parts of 



