28 FIXING AND HARDENING. 



the volume of liquid be insufficient, its composition will soon become 

 seriously altered by the diffusion into it of the soluble substances of 

 the tissues ; and the result may be a macerating instead of a harden- 

 ing liquid. Further, as soon as, in consequence of this diffusion, the 

 liquid has acquired a composition similar in respect of the propor- 

 tions of colloids and crystalloids contained in it to that of the liquids 

 of the tissues, osmotic equilibrium will become established, and 

 diffusion will cease ; that is to say, the hardening liquid will cease 

 to penetrate. This means, of course, maceration of internal parts. 

 On the other hand, it appears that a certain slight proportion of 

 colloids in the hardening liquid is favourable to the desired reaction, 

 as it gives a better consistency to the tissues by preventing them 

 from becoming brittle. Hence the utility of employing a certain 

 proportion of hardening agent. 



Hardening had better be done in tall cylindrical vessels, the 

 objects being suspended by a thread, or muslin bag, or otherwise, 

 at the top of the liquid. This has the advantage of allowing diffusion 

 to take place as freely as possible, whilst any precipitates that may 

 form fall harmlessly to the bottom ; or, they may be laid on a layer 

 of cotton- wool, or filter-paper, or spun glass. 



In general, begin hardening with a weak reagent, increasing the 

 strength gradually, as fast as the tissues acquire a consistency that 

 enables them to support a more energetic action of the reagent. 



Let the objects be removed from the hardening fluid as soon as 

 they have acquired the desired consistency. 



