44 HARDENING AGENTS. 



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 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 favorable 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 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. 



69. 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. 



70. Chromic Acid. Chomic acid is generally employed in 

 strengths of |-th per cent, to J 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, Kanvier) . 



In order to obtain the best results, you should not employ 

 portions of tissue of more than an inch cube. For a human 

 spinal cord, you should take two litres of solution, and change 



