20 LABORATORY GUIDE IN BACTERIOLOGY 



the above proceeding will accomplish the object. If it is desired 

 to have a medium which has a reaction of r per cent acid, a pro- 

 portionate amount should be deducted from the total amount of 

 NaOH calculated. 



Example. — By reading the burette we find that it takes 3 c.c. 

 i-20th normal NaOH to neutralize s c.c. of the medium. This 

 means that 60 c.c. i-2oth n.NaOH will neutralize 100 c.c. medium, 

 or 600 c.c. i-2oth n.NaOH will neutralize 1,000 c.c. medium. 

 To find the requisite amount of n.NaOH divide the above figure 

 by 20. Then. 3 c.c. n.NaOH will neutralize 100 c.c. and 30 c.c. 

 n.NaOH will neutralize 1,000 c.c. If the reaction is to be 

 I per cent acid, deduct 10 c.c. from 30 = 20 c.c. If 20 c.c. 

 normal NaOH are added to each liter the reaction should be i per 

 cent acid. This should be ascertained by a second titration. 



A normal solution is the equivalent weight in grams (Gram- 

 Molecule) dissolved in distilled water and made up to 1,000 c.c. 

 In the case of monovalent chemicals the molecular weight is 

 taken, if bivalent the molecular weight is divided by two, etc. 



All media slioxild be prepared with the utmost care 

 and should be perfectly clear. 



EXERCISE 2. PREPARATION OF NUTRIENT AGAR- AGAR 



Agar-agar (or called simply "agar") is a watery 

 extract of certain seaweeds found on the Pacific coast 

 of Asia. A solution of agar containing about i . 5 per 

 cent forms a firm jelly, which melts near the boiling 

 point of water, and on cooUng solidifies at about 39°. 

 Gelatin solidifies at much lower temperature, and can- 

 not be kept soUd at body temperature. The use of 

 agar is, therefore, of great importance in the study of 

 pathogenic bacteria, a large number of which prefer 

 body temperature for growth. 



1. Weigh a saucepan, or, if available, a double 

 boiler. Note the weight. 



2. Measure 1,000 c.c. of tap water into the sauce- 



