SOME HARDENING AGENTS. 379 



wash out picric acid or bichloride of mercury with water ; always 

 use alcohol. 



The picro-suli)huric acid has great penetrating powers, and is 

 totally soaked out of the structure with alcohol, leaving them in a 

 good condition to stain. If much lime is in the tissues it is not to 

 be recommended, for it dissolves out the lime and throws it down 

 as crystals of gypsum in the tissues. For such the picro-nitric or 

 picro-hydrochloric acid is to be preferred (Mayer's formula). 



Bichloride of Mercury. — The other agent is a most rapid 

 hardening one, so that tissues must not be left in it for too long 

 a time. For small pieces, a quarter of an hour or thereabouts is 

 sufficient ; for large pieces, one to two hours. The pieces when 

 fixed become whitish throughout. 



For glands and glandular structures generally, a half-saturated 

 alcoholic solution is most useful — /,^., to 50 cc. of a saturated alco- 

 holic solution add 50 cc. of 70 per cent, alcohol. Vignal recom- 

 mends that to TOO cc. of this mixture there be added five to six 

 drops of nitric acid. The pieces of glands should be cut into 

 cubes about four millimetres in size, and can be hardened in an 

 hour or so. Finish in alcohol. I object to this agent as a general 

 one, for it contracts so much, and it does not bring out the finer 

 structural details, as the striae in the muscles of the tongue, etc. 



A saturated watery solution contains about 5 per cent, of this 

 salt ; but it is much more soluble in alcohol, especially alcohol of 

 50 to 60 per cent. Keep both a saturated watery and a saturated 

 alcoholic solution on hand. The salt will hasten hardening if the 

 fluid is heated to 30° C, and all stains can readily be used after it. 

 Specimens, unfortunately, after bichloride hardening, if left in 

 alcohol for any length of time, become brittle. It is best to stain 

 and embed in paraffin, where they can be kept for some time, 

 either dry or in alcohol. 



Dilute Nitric Acid (such as Altmann's method) is very useful 

 and applicable to most delicate objects in histology, even to the 

 retina. It is claimed to be the most trustworthy fixing agent for 

 protoplasm, but if so, why is it not more used ? So far, I believe 

 it is chiefly used in embryology by a few special investigators. A 

 secondary point is that it is one of the best dissociating and 

 decalcifying agents. 



