252 E. CRESSWELL BABER. 



to test its goodness when emerging from the filter ; this is 

 done in the following manner : — Place a drop of the solution 

 on to a piece of white filtering paper, and allow it to dry, 

 when, if the picro-carminate be good, a yellow spot is formed 

 surrounded by a distinct red ring. 



Observers' differ in the exact shade of yellow or red, Avhicli 

 they prefer, but if, with the fluid prepared as above, the two 

 colours be distinctly marked, and the solution sufficiently 

 concentrated, there can be no doubt about its goodness. 

 If the picric acid be in excess the yellow colour will prepon- 

 derate over the red, and vice versa. Should the solution be 

 too dilute, as indicated by the colours being very faint, it 

 may be strengthened by concentration, by the digestion in 

 it of more of the desiccated powder, or by repeated filtra- 

 tion through the same filter, by which latter method more of 

 the picro-carminate is taken up by the water each time. 



In order to be sure of having the right proportion of the 

 ingredients to start with, it is as well to apply this test also 

 to the mixture before drying, as after desiccation the relative 

 proportion of the picric acid, carmine, and ammonia cannot be 

 altered without evaporating to dryness again. 



Specimens can be put from water into the staining fluid, 

 but from this, if the full coloration of the picro-carminate is 

 required, they must be put directly into glycerine, for, if placed 

 first in water, the picric-acid is dissolved out. If, on the 

 other hand, the staining by carmine only is desired, the 

 preparation is passed through water before being put in 

 glycerine, and it is said that the colouring thus obtained is 

 more regularly disposed than that obtained by ordinary 

 carmine. 



As the picric acid does not fix itself to the tissues like the 

 carmine, in order that the yellow colour that it produces be 

 permanent, it is necessary to add a small quantity of picro- 

 carminate to the glycerine in which the specimen is 

 preserved. 



The following method of staining and mounting is con- 

 venient : — The section is placed on a glass slide and a drop 

 of the colouring fluid dropped on it ; after a few minutes, 

 when the specimen is seen to be sufficiently stained, the 

 cover glass is put on, and some of the following mixture 

 drawn under by means of filtering paper — 



Solution of Picro-Carminate ... 1 drop. 



Glycerine 10 drops. 



Water 10 drops. 



The specimen can then be sealed up. The exact propor- 



