32 FIXING AND HARDENING AGENTS 



fresh after a few days. Six weeks or two months are necessary 

 to complete the hardening. 



Lee thought it was frequently useful to add a little glycerin ; 

 there is then less brittleness. 



The solution should be taken weak at first, and the strength 

 increased after a time. The objects should be removed from 

 the solution as soon as they have acquired the desired consistency, 

 as if left too long they will become brittle. They may be pre- 

 served till wanted in alcohol (95 per cent.). It is well to wash 

 them out in water for twenty-four or forty-eight hours before 

 putting them into the alcohol. After a time they generally become 

 green in the alcohol. They may be bleached if desired. 



Cliromic acid is a most powerful and rapid hardening agent. 

 (By it you may obtain in a few days a degree of hardening that 

 you would hardly obtain in as many weeks with bichromate 

 for instance.) It has the defect of a great tendency to cause 

 brittleness . 



44. Chromo-acetic Acid (Flemming, Zellsbz., Kern. u. Zellth.. p. 382). 



Chromic acid . . 0-2 to 0-25 per cent, in water with, 



Acetic acid . . 0-1 per cent. 



Flemming found this the best reagent for the study of the achromatic 

 elements of karyokinesis. You can stain with hsematoxylin, or the 

 basic anilin dyes. 



The following has been recommended for Annelids by Ehlers : — 

 To 100 c.c, of chromic acid of 0-5 to 1 per cent, add from 1 to 5 drops 

 of glacial acetic acid. The acetic acid is said to be sufficient to counter- 

 act any shrinkage due to the chromic acid. Fix overnight, wash out 

 several hours in water. 



Similar to this is the " chromo-acetic acid, No. 1," of Lo Bianco 

 (Mitth. Zool. Stat. Neapel, ix, 1890, p. 443), viz. 1 part 50 per cent, 

 acetic acid and 20 parts 1 per cent, chromic acid, which is found very 

 useful for fixing marine animals. 



45. Chromo-nitric Acid (Perenyi's formula, Zool. Anzeig., v, 

 1882, p. 459). 



4 parts 10 per cent, nitric acid, 



3 parts alcohol. 



3 parts 0-5 per cent, chromic acid. 



These are mixed, and after a short time give a fine violet- 

 coloured solution. 



The objects are immersed for four to five hours, and then 

 passed through 70 per cent, alcohol (twenty-four hours) strong 

 alcohol (soine days), absolute alcohol (four to five days). They 

 are then fit for cutting. The advantage of the process is, amongst 

 others, that segmenting eggs and nuclei are perfectly fixed, the 

 ova do not become porous, and cut like cartilage. 



Chromo-nitric acid is not only an embryological reagent, 

 and a very important one, but also an admirable one for general 



