METHODS. 9 



3. Picro-carmine (or ammonium picro-carminate) is another 

 useful preparation of carmine. It is prepared by adding a strong 

 ammoniacal solution of carmine to a quantity of concentrated 

 solution of picric acid in water, until a precipitate begins to be 

 formed ; it is then evaporated to about one-fifth of its bulk 

 filtered, and the filtrate is evaporated to dryness. The crystal- 

 line residue is dissolved in water so as to make a 5 per cent 

 solution, and this may be diluted as occasion requires. 



Another method (Gage) is to dissolve a quantity of picric acid 

 in ] 00 parts of water, and an equal quantity of carmine in 50 

 parts of solution of ammonia ; these are then mixed, filtered, 

 evaporated to dryness, and the residue dissolved in 100 parts 

 of water. 



Picro-carmine is used especially for staining nuclei, the staining 

 being more uniform than when carmine alone is used : it has this 

 further advantage, that a prolonged exposure to it does not pro- 

 duce overstaining, as is the case with the other preparations 

 of carmine. The objects should be previously kept for some 

 time in absolute alcohol. If it is desired to retain the double 

 staining which this reagent produces, the sections must be 

 mounted at once in glycerine ; but if the carmine staining only 

 is required, the sections must be washed in water, which will 

 dissolve out the picric acid. When stained sections are mounted 

 in glycerine, a small quantity of picro-carmine must be added 

 to the glycerine in order to preserve the colours. 



The various preparations of carmine can be used as well for 

 tissues which have been hardened in chromic, picric, or osmic 

 acid, as for fresh tissues, but the former stain less readily. 



4. Cochineal. The ordinary preparations of carmine frequently 

 fail to give good results, especially when the tissue has been 

 previously treated with chromic acid. Other preparations of 

 the same colouring matter made directly from the cochineal 

 insect have therefore been employed. 



1. Alcoholic Solution. A quantity of finely powdered cochi- 

 neal (best grey) is extracted for several days with 70 per cent, 

 alcohol ; the liquid is filtered off and is ready for use. 



2. Solution in water. Seven grammes of cochineal and an equal 

 quantity of burnt alum are rubbed up together in a mortar until the 

 whole is a fine powder : the powder is then added to 700 c.c. of 



