220 INJECTIONS. 



changes from ammoniacal to sour. As soon as the sour smell 

 is perceived, the addition of acetic acid must cease, and the 

 liquid be examined under the microscope. If it contains a 

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

 and the mass must be thrown away. 



Eanvier states that by practice the operator learns to attain 

 to perfect neutralisation almost infallibly in this way ; and 

 that this is the only way to attain to it. Trust must not be 

 put in certain formulae that profess to indicate the propor- 

 tions of ammonia and acetic acid necessary for neutralisation, 

 on account of the variation in strength of the solutions of am- 

 monia kept in laboratories. The method proposed by Prey of 

 determining beforehand the quantity of a known acetic so- 

 lution that is necessary for neutralisation of a given quantity 

 of the ammonia employed, is not infallible because it often hap- 

 pens that commercial gelatin is acid ; in which case the pro- 

 posed method would cause the operator to overpass the point 

 of saturation. 



The mass having been perfectly neutralised is strained 

 through new flannel. 



440. How to Neutralise a Carmine Mass (VILLE, Gaz. hebd. 

 d. Sci. med. de Montpellier, Fev., 1882 ; may be had separately 

 from Delahaye et Lecrosnier, Paris). Ville is of Kanvier's 

 opinion that the method of titration recommended by Frey is 

 defective, but for a different reason. When carmine is treated 

 with ammonia a certain proportion of the ammonia combines 

 with the carmine to form a transparent purple compound, and 

 the rest of the ammonia remains in excess. It is this excess 

 that it is required to neutralise precisely. In Frey's method a 

 quantity of acid sufficient for the neutralisation of the whole 

 of the ammonia employed is taken ; hence, naturally, the point 

 of neutralisation is over-stepped, and a granular mass is the 

 result. 



As to the acidity accidentally found in commercial gelatin, 

 that source of error is easily eliminated. Instead of soaking 

 the gelatin in water, it should be placed in a large funnel with 

 a narrow neck, or better, in a stop-cock funnel, and the whole 

 should be placed under a tap, and a stream of water arranged 

 in such a manner that the gelatin be constantly completely 

 immersed. Washing for an hour or so in this way will 



