LABORATORY AND TEACHING METHODS 613 



Take three pots A, B, C, which have been thoroughly sterilized by dry heat in 

 a sterilizing oven. Place in pot A ordinary rich garden soil. Pill pot B and C with 

 sand and thoroughly sterilize both pot and sand with dry heat. Plant in pots, A, B 

 and C seeds of pea, bean, clover or those of other leguminous plants and water 

 pots A, and B only with distilled water previously carefully sterilized. Pot C with 

 sand, is watered with distilled water which has been allowed to percolate through 

 rich garden earth and which removes the bacterial life which such rich soil con- 

 tains. Pot C watered with such water, therefore, becomes microbe-seeded. After 

 the first watering, all subsequent applications of water should be made with 

 thoroughly sterilized distilled water. 



Note daily the growth of the plants in each of the pots and explain the difference 

 in the rate and character of the growth, if any. 



In order to be able to study microscopically the entrance of the organisms from 

 the soil into the root of the leguminous plants a larger series of pots should be used 

 than three. By doing this successive stages in the development of the nodules can 

 be obtained and made ready for microscopic study by the paraffin method described 

 in a subsequent lesson (Page 656). 



LESSON 18 



Standardization of Culture Media (F. D. Heald). — Bacteria and fungi are in- 

 fluenced in their development by the degree of acidity or alkalinity of the medium 

 in which they are growing. Since this is true, it is important to employ media of 

 known reaction. In order to secure results which may be compared, the adoption 

 of a uniform method of standardization is necessary and the reaction of a culture 

 medium should be indicated always when cultural or morphologic characters are 

 described. The standardization of culture media requires the following solutions: 



N 

 -J „„ = a normal solution of sodium hydroxide. 



N 



— NaOH = twentieth normal solution of sodium hydroxide. 



20 



N ... 



vfp,-; = normal hydrochloric acid. 



N N 



The — NaOH is used for the titration of culture media and the iv-FiTSF for their 

 20 JNaUxl 



N 

 neutralization. TTpi is used for acidifying media. A normal solution contains 



1 gram of basic H, or the equivalent to each 1000 c.c. Since the above normal solu- 

 tions are required in every pathologic laboratory, directions are here given for their 

 preparation. 



Preparation of Normal Solutions. — Normal solutions of NaOH or HCl cannot be 

 made by weighing. NaOH readily absorbs CO2 and water from the air and so can- 

 not be weighed accurately enough for making standard solutions. HCl is liquid 

 and of varying strength. It is necessary, then, to start with an acid or alkali that 

 is in solid crystalline form and is not altered on exposure to the air. Oxalic acid 

 presents the requisite characteristics. 



