40 METHODS OF MICROSCOPICAL RESEARCH. 



immersion may be necessary for others. No tissue 

 should be allowed to remain in chromium fluids 

 until it is quite hard, if, as is commonly the case, 

 it is necessary to complete the hardening in alcohol 

 to which it should be removed when it has become 

 tough, though still retaining its elasticity. When 

 transferring a tissue to alcohol to complete its 

 hardening, all trace of the medium in which it has 

 already been immersed must be removed by pro- 

 longed soaking in cold water, which should be 

 frequently changed during twelve or twenty-four 

 hours, or until the last washings shall be colourless. 

 Tissues thus prepared must not be put at once into 

 full strength spirit, but first placed in methylated 

 spirit and water ("half and half") for twenty-four 

 hours, thence into pure methylated spirit. It is 

 most important that the shrinkage of the tissues 

 caused by the hardening processes should be 

 uniform as well as gradual; it is therefore again 

 obvious that the greatest care must be taken to 

 use fluids which will penetrate the whole tissue or 

 organ, lest the surfaces merely should become 

 hardened whilst the interior portions remain soft 

 and even become decayed. 



As a rule it is advisable to commence with weaker, 

 and to advance by degrees to stronger, solutions in 

 order to ensure penetration. A book should be 

 kept in which notes can be made of the nature of 

 the fluids in which organs and tissues are hardening, 

 the dates of changing or substituting fluids, &c. 

 Each bottle should bear a letter or number to corres- 

 pond with the letter or number registered against 

 it in the note book. Tissues and organs may be 

 wrapped in pieces of linen, and a label bearing its 



