260 CHAPTER XXI. 



soon as the sour smell is perceived the liquid must be exa- 

 mined under the microscope. If it contains a granular 

 precipitate of carmine, too much acid has been added, and 

 it must be thrown away. 



The mass, having been perfectly neutralised, is strained 

 through new flannel. 



488. How to Neutralise a Carmine Mass (VILLE, Gaz. held. d. 

 Sci. med. de. Montpellier, Fev., 1882; maybe had separately 

 from Delahaye et Lecrosnier, Paris). VILLE points out that 

 when carmine is treated with ammonia a certain proportion 

 of the ammonia combines with the carmine and the rest 

 remains in excess. It is this excess that it is required to 

 neutralise precisely, not the whole of the ammonia employed. 



To neutralise the acidity of commercial gelatin, it should be 

 washed for an hour or so in running water. 



As to the neutralisation of the colouring mass, VILLE is of 

 opinion that the sour smell cannot be safely relied on in 

 practice, and prefers to employ dichroic litmus paper (litmus 

 paper sensitised so as to be capable of being used equally 

 for the demonstration of acids and bases). For directions 

 for preparing this see loc. cit. or previous editions. 



489. HOYER'S Carmine-Gelatin Mass (Bid. Centralb., 1882, 

 p. 21). Take a concentrated gelatin solution and add to it 

 the needful quantity of neutral carmine staining solution (loc. 

 cit., p. 17). Digest in a water-bath until the dark violet-red 

 colour begins to pass into a bright red tint. Then add 510 

 per cent, by volumes of glycerin, and at least 2 per cent, by 

 weight of chloral, in a concentrated solution, and strain. 



490. FOL'S Carmine-Gelatin Mass (Lehrl., p. 13). This can 

 be kept in the dry state for an indefinite length of time. 



Gelatin in sheets is cut into strips which are macerated 

 for two days in carmine solution (prepared by diluting 

 one volume of strong ammonia with three of water and 

 adding carmine to saturation, and filtering after a day 

 or two). The strips are then rinsed and put for a few 

 hours into water acidulated with acetic acid, then washed on 

 a sieve for several hours in running water, dried on parch- 

 ment paper, or on a net, and preserved for future use. To 



