102 CARMINE AND COCHINEAL STAINS. . 



acid of 45 per cent, strength add carmine until no more will 

 dissolve, and filter. (Forty-five per cent, acetic acid is the 

 strength that dissolves the largest proportion of carmine.) 



To use the solution you may either dilute it to 1 per cent, 

 strength, and use the dilute solution for slow staining, which 

 is the method to be preferred for making glycerin prepara- 

 tions ; or a drop of the concentrated solution may be added to 

 a fresh preparation under the cover-glass. 



This is a very important reagent, which in certain cases 

 renders services that no other reagent can render. If you use 

 the concentrated solution it fixes and stains at the same time, 

 and hence is most valuable for the study of fresh objects. It 

 is very penetrating, a quality that enables it to be used where 

 ordinary reagents would totally fail. The stain is a pure 

 nuclear one. Unfortunately the preparations cannot be pre- 

 served, and for this and other reasons the stain is of very 

 restricted application. 



ZACHAEIAS (see Zeit.f. wiss. Mik., v, 3, 1888, p. 371) adds 

 to this solution wood vinegar (acetum pyrolignosum) in the 

 proportion of 1 drop to 10 c.c. 



j3. So-called " Neutral" and Alkaline. 



159. As to Picro-carmine. The term " picro-carmine " is commonly 

 used to denote a whole tribe of solutions in which carmine, ammonia, and 

 picric acid exist uncombined in haphazard proportions. EANVIEE, to whom 

 we owe the invention of picro-carmine, claims that when prepared by his 

 process it results as a definite chemical substance, a double salt of picric and 

 carminic acid and ammonia, or pier o -car minate of ammonia. I am not 

 prepared to say whether this is or is not the case, but I think it may be 

 safely said that Ranvier's method of preparation is the only one yet pub- 

 lished that will give with certainty a product having the desired qualities. 



Prepared in this way picro-carmine gives very delicate differentiations, and 

 has the great merit of being less hurtful to most tissues than other aqueous 

 alkaline carmines. It is a single or a double stain according to the manner 

 of using it. If the preparations be washed, after staining, with water, it is 

 a single stain, the colour of the carmine alone appearing ; if they be washed 

 quickly in alcohol it is a double stain, the yellow coloration of the picric 

 acid not being dissolved by the alcohol as it is by water. Of course the 

 washing with alcohol must not be overdone, or the yellow coloration may 

 be entirely removed. It should be understood that the chief value of picro- 

 carmine does not lie in its capacity of affording a double stain. The double 

 stain, if that is all that is wanted, can be just as well or better obtained by 

 staining first with borax-carmine, or the like, and after-staining with picric 

 acid. The essential point about picro-carmine is that it is a fairly neutral 



