TECHNIC BACTERIOLOGICAL EXAMINATION OF SOILS. 289 



formity and so that results obtained from either higher or lower dilutions of 

 other materials would be more comparable, every higher bacterial dilution 

 should be ten times the one from which it is made. Thus there would naturally 

 be two uniform systems of making dilutions of the bacteria in soil that might 

 reasonably be emploj'ed; either 1 ce of the lower bacterial dilution and 9 cc 

 of sterile water to make the next bacterial dilution or 10 ce of the lower 

 bacterial dilution and 90 cc of sterile water to make the next bacterial dilu- 

 tion. It would be necessary in either ease to make as many dilutions as ten 

 is a factor of the highest dilution desired. 



Many investigators take one cc of the first mixture made up and put it 

 with 99 cc or 199 cc of sterile water, making the resulting bacterial dilution 

 one hundred times, or two hundred times as great at once. 



Plate V. 



To determine whether a one cc aliquot of a soil and water mixture would 

 be as representative as a ten cc aliquot the following experiments were carried 

 out. Dry and sieved samples were used as counts were not desired. Two 

 acid soils were chosen, No. 1, an acid black sand and No. 2 a very acid peat. 

 50 grams of each soil were taken in each ease, put with 200 cc of water and 

 shaken for five minutes. A one cc portion of each mixture was taken with 

 a one cc pipette while the mixture Avas still in motion and put with 99 ee of 

 distilled water and shaken. Ten cc ahquots of the original mixtures were 

 taken in the same manner as the one cc aliquots, put with 90 cc of water and 

 shaken. This gave a 1-40, and 1-400 bacterial dilution of each soil. Ten cc 

 aliquots of the 1-40 bacterial dilutions were put with 90 cc of distilled water 

 giving 1-400 bacterial dilutions. Thus two 1-400 bacterial dilutions of each 

 soil were made, one on the 10 cc basis and one on the 1 cc basis. Using 10 cc 



