NEUTRALISATION OF MEDIA 31 



XaOH than litmus neutral media, and if for the neutralisation not 

 caustic soda but sodium carbonate is used the discrepancy becomes 

 still greater. For some years past we have ourselves adopted in 

 our investigations a standard reaction midway between the neutral 

 point of litmus and phenolphthalein, but a series of experiments, 

 lately undertaken, has convinced us that an equally favourable 

 reaction for the growth of all micro-organisms likely to be found in 

 milk is a point at which the first appearance of a pinkish tinge can 

 be observed in a dilution of the medium when at the boiling point, 

 on the addition to it of a few drops of a 50 per cent alcoholic 

 solution of phenolphthalein. This reaction has the advantage of 

 representing a very exact point, and it is, with an ordinary' amount 

 of care, so easy of attainment that we strongly recommend its 

 adoption for all media, except those in which a certain degree of 

 acidity is desired. 



In regard to this question of standard media we propose 

 to make reference to two chief points — {a) neutralisation of media, 

 {b) peptones. 



Neutralisation of Media 



This may be looked upon as the most important stage in the 

 preparation of media, and too much care cannot be exercised if 

 accurate results and a constant standard of reaction is to be 

 obtained. One of two methods may be adopted. 



1. Tlie fractional method, as originally recommended in the 

 report of the Bacteriological Committee appointed by the American 

 Public Health Association issued in the year 1 897 ; and 



2. T/i€ metJiod of neutralisation in bulk. — Of the two, the 

 authors, after experience of both, prefer the latter as simpler, less 

 liable to error, and equally accurate in results, but as the former 

 has been generally adopted in America, and, with various modifica- 

 tions, in certain of the schools of this and other European countries, 

 both are given. 



Fractional method^ — 



The following solutions are required : — 



1. A 05 per cent, solution of commercial phenolphthalein in 

 50 per cent alcohoL 



2. A oV normal solution of sodium hydrate. 



3. A 2Tr normal solution of HCl. 



* Reports and Papers of the American Public Health Association^ vol. xxiii., 

 lietseq. 



