HARDENING AGENTS. 



The subject of hardening agents is one of very great import- 

 ance and presents a field in which much yet remains to be learned. 

 The object of hardening is to bring the tissue in a condition to be 

 either sectioned directly without crushing, or to allow it to be 

 infiltrated with some substance that will hold it firm for cutting. 

 The difficulty to overcome is to find an agent that will harden the 

 tissue without shrinking it. The method employed to overcome 

 this is to bring the tissue in contact with the dilute hardening 

 agent, and then gradually increase its strength to prevent a vio- 

 lent action between it and the tissue. The former must always be 

 present in a large excess in order that an equilibrium may not be 

 established too quickly. 



The exact strength of the hardening agent in which the tissue 

 should first be placed is a matter of some uncertainty and can only 

 be determined after experimenting with each sort of tissue. After 

 the object is hardened, it should not be left any great length of 

 time in the full strength of the hardening agent as it is liable to 

 become brittle. 



In arranging the tissue to be hardened it should be carefully 

 trimmed and only the portions that are needed for examination 

 placed in the agent. In the case of large pieces, as, for example, 

 closed pistils, cut the parts open to allow the hardening agent to 

 penetrate all parts of the object, otherwise, deterioration will result. 



Alcohol. 



This is one of the most frequently employed hardening agents, 

 and for many plant tissues is all that could be desired. For most 

 soft material 40 per cent, alcohol is dilute enough to begin the 



