CHEOMO-ACETIC ACID. 21 



follow chromic acid is haematoxylin, or, for sections, some 

 anilin stain. But the previous washing out with water must 

 be very thorough if good results are to be insured ; it may 

 take days. 



Chromic acid is not a very penetrating reagent, and for this 

 reason, as well as for others, is seldom used pure, but plays an 

 important part in the mixtures described below, of which the 

 chief is certainly the mixture of Flemming. A chief objection 

 to the use of chromic acid is that it precipitates certain of the 

 liquid albuminoids of tissues in the form of filaments or net- 

 works, which are often of great regularity, and simulate struc- 

 tural elements of the tissues. This objection applies to all 

 mixtures into which chromic acid enters. 



Action of light on alcohol containing chromic objects. When 

 objects that have been treated by chromic acid or a chromate 

 are put into alcohol for hardening or preservation, it is found 

 that after a short time a fine precipitate is thrown down on the 

 surface of the preparations, thus forming a certain obstacle to 

 the further penetration of the alcohol. Previous washing by 

 water does not prevent the formation of this precipitate, and 

 changing the alcohol does not prevent it from forming again 

 and again. It has been found by Hans Virchow (Arch.f. mik. 

 Anat. } Bd. xxiv, 1885, p. 117) that the formation of this precipi- 

 tate may be entirely prevented by simply keeping the pre- 

 parations in the dark. The alcohol becomes yellow as usual 

 (and should be changed as often as this takes place), but no 

 precipitate is formed. If this precaution be taken, previous 

 washing with water may be omitted, or at all events greatly 

 abridged. 



The brownish-green colour of chromic objects may be re- 

 moved by treating them with peroxide of hydrogen (Unna, in 

 Arch. f. mik. Anat., Bd. xxx, 1887, p. 47; cf. Journ. Roy. Mic. 

 Soc., 1887, p. 1060). 



31. Chrome-acetic Acid (FLEMMING, Zellsbz. Kern. u. Zellih. y 

 p. 382). 



Chromic acid . 0'2 to 0'25 per cent. 

 Acetic acid . . O'l per cent., in water. 

 Flemming finds this the best reagent for the study of the 

 achromatic elements of karyokinesis. (Flemming wrote this in 

 1882, and I doubt whether it would now hold good.) Stain 



