46 HARDENING AGENTS. 



a macerating instead of a hardening liquid. Further, as soon 

 as, in consequence of this diffusion, the liquid has acquired a 

 composition similar in respect of the proportions of colloids 

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

 tissues, osmotic equilibrium will become established, and dif- 

 fusion will cease. That is to say, the hardening liquid will 

 cease to penetrate. This means, of course, maceration of in- 

 ternal parts. On the other hand, it appears that a certain 

 slight proportion of colloids in the hardening liquid is favor- 

 able 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 harden- 

 ing agent. 



Hardening had better be done in tall cylindrical vessels, the 

 objects being suspended by a thread 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. 



Always begin hardening with a weak reagent, increasing 

 the strength gradually, as fast as the tissues acquire a con- 

 sistence 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. 



As to the choice of a hardening reagent, if you wish, above 

 all, for a rapid and energetic action, take chromic acid. If 

 you wish for a more moderate and more equable action, take 

 a chromic salt, or one of the compounds of which the chromic 

 salts are the principal ingredients. 



Mineral Acids. 



69. Chromic Acid. Chromic acid is generally employed in 

 strengths of -J-th per cent, to \ per cent., the immersion lasting 

 a few days or a few weeks, according to the size and nature 

 of the object. Mucous membrane, for instance, will harden 

 satisfactorily in a few days; brain will require some six 

 weeks. 



Large quantities of the solution must be taken (at least 200 

 grammes for a piece of tissue of 1 centimetre cube, Ranvier) . 



In order to obtain the best results you should not employ 



