HOW TO NEUTRALISE A CAKMTNE MASS. 221 



remove all traces of acids mechanically retained in the 

 gelatin. 



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

 opinion that the criterion of neutrality given by Ranvier the 

 sour smell that takes the place of the ammoniacal odour can- 

 not be safely relied on in practice. He considers it greatly 

 preferable to employ dichroic litmus paper (litmus paper 

 sensibilised so as to be capable of being used equally for the 

 demonstration of acids and bases). 



To prepare such a paper, the tincture obtained by decoction 

 of cake litmus is slightly acidified by an excess of sulphuric 

 acid. By this means the excess of alkali, or of alkaline 

 carbonates, that is always present in litmus decoction, and 

 which diminishes its sensibility as a reagent, is neutralised. 

 The decoction is then heated and agitated with an excess of 

 precipitated carbonate of baryta, and filtered. 



The solution of litmus thus obtained is exposed to the air 

 in wide vessels until its intense blue colour has given place to 

 a reddish tint. Strips of white unsized paper are then 

 dipped in it, and dried in the shade on stretched threads, in 

 a place free from vapour of ammonia. 



A shorter method consists in adding very dilute sulphuric 

 acid, drop by drop, to the ordinary laboratory tincture of litmus, 

 until the colour changes to red. Then, by adding successively 

 traces of alkali and very dilute sulphuric acid, the reddish, 

 dichroic tint may be obtained, and the paper prepared with 

 the solution as before. 



The paper is used in the same way as ordinary litmus paper. 

 A strip is moistened with distilled water and held as close as 

 possible to the injection mass kept melted on a water-bath. 

 It becomes blue at first, very rapidly and decidedly ; but as 

 fast as fresh quantities of acid are added, this reaction 

 becomes less evident, and at a certain moment the change of 

 colour becomes very slow in making its appearance. It is then 

 that the addition of acid should cease, and the operation is 

 ended. 



Very delicate sensitised paper may also be prepared with 

 other reagents than litmus, for instance, withNessler's reagent* 



* Nessler'a reagent may be prepared as follows : Mercuric chloride, in 

 powder, 35 grammes ; iodide of potassium, 90 grammes ; water, 1750 c.c. 

 Heat gently till dissolved in a large basin ; then add of stick caustic potash 



