29 o CLEAN MILK 



as strong as N/ 20 NaOH) will be required to neutralize ioo c.c.s of 

 the infusion (twenty times as much as the five c.c.s actually tested). 

 Since 2.5 c.c.s normal acid are (theoretically) necessary to 

 neutralize 100 c.c.s of the infusion (in the case under considera- 

 tion), the reaction of the infusion would be stated as equivalent 

 to 2.5 c.c.s of normal acid per 100; or, briefly, "the infusion is 

 2.5% acid." 



(f) The addition of the peptone (Step 3) scarcely alters the 

 acidity as a rule: nor do the constituents mentioned under 5(a) 

 and 5 (b) ; but the addition of gelatin 5 (c) and the action of B. coli 

 ! 5(d) add considerably to the original acidity, as a rule. Hence 

 the necessity for Step 6, which is thus performed: (1) Determine 

 the existing acidity; suppose it to prove 2.8% acid. (2) Adjust 

 to 1.5% acid thus: 2.8 — 1.5 = 1.3; hence add to the medium 1.3 

 c.c.s normal alkali per 100 c.c.s. Theoretically, titration of the 

 medium should now show 1.5% acid. In practice the acidity will 

 prove to be higher, say 1.8%. As the desired reaction is 1.5 this 

 means that (1.8 — 1.5) .3 c.c.s normal alkali per 100 c.c.s medium 

 must still be added. Usually titration after this addition will yield 

 'the desired 1.5% acidity. If the acidity be still too high (say 

 1.6 then 1.6 — 1.5 = .1) .1 c.c.s normal alkali per 100 c.c.s media 

 must be added, and titration now will show that the desired point 

 has at last been reached. Should any accident such as the ad- 

 dition of too much normal alkali occur, by which the medium is 

 adjusted to a point below the point sought (?'. c, I'y'c acid, when 

 1.590 is desired) the difference between the existing acidity and 

 the acidity sought will indicate the (theoretical) amount of normal 

 acid to be added per 100 c.c.s of media, to restore it to the desired 

 acidity (i. c, .5 c.c.s normal acid in the case quoted). But media 

 in the preparation of which any such departure from the regular 

 course occurs should not be used for official or research work on 

 counts. 



Note : Normal NaOH is prepared in such a manner as to secure 



