MATERIAL FOR RED INJECTION. 163 



of water are added. When the carmine is as com- 

 pletely as possible dissolved, the liquid is filtered. 

 The process of filtration occupies some time several 

 hours, in fact and may be conveniently left over- 

 night. The filtrate is warmed, and the gelatine solu- 

 tion is gradually added to it with constant stirring. 

 The next part of the process is to precipitate the 

 carmine, for otherwise it would difl'use through the 

 walls of the vessels and color the tissues ; but it must 

 he precipitated so finely that the particles shall not 

 be visible even under the highest power of the micro- 

 scope. To effect this object a small quantity of a ten 

 per cent, solution of acetic acid is placed in a burette 

 and allowed to run drop by drop into the warm car- 

 minized, gelatine solution, which is all the while 

 constantly agitated. As its alkalinity becomes neu- 

 tralized the amrnoniacal odor becomes less and less 

 strong, and eventually disappears, and is replaced by 

 the vinegar-like smell of acetic acid. The alteration 

 in reaction may be shown, in spite of the red color 

 of the solution, by placing a small drop on a piece of 

 blue litmus paper ; if the opposite side of the paper be 

 looked at, it will be found to have assumed the 

 characteristic bright-red color which acids produce, 

 and which is quite different from carmine. This 

 change is owing to the diffusion of the acetic acid 

 through the paper, whereas the carminized gelatine 

 sets almost immediately and is thus unable to soak 

 through. 



But it is not sufficient in order to effect the pre- 

 cipitation of the carmine that the fluid should only 

 just be acidulated ; there must be an excess of acid. 

 A few more drops are therefore added, and the car- 

 mine thrown out of solution. This change from 

 the soluble to the insoluble state is accompanied by 

 a very marked alteration in color, for whereas whilst 

 still in solution the carmine imparted the rich deep 

 red of an ammoniacal solution to the gelatine, after 

 the precipitation the color of the latter changes to a 

 paler red, comparable rather to the tint presented by 



