54 THE AMERICAN MONTHLY [Feb 



The numerical expressions given in the following table 

 may be considered as the approximate equivalents of the 

 terms now employed for expressing low turbidities. 



Low turbidities observed in this manner are not appre- 

 ciably interfered with by the color of the water, and con- 

 sequently the color standards need not to be used in con- 

 nection with the turbidity standards. But above a tur- 

 bidity of twenty the water in the gallon bottles is too 

 opaque to make satisfactory comparisons, and it becomes 

 necessary to resort to the use of 100 cc tubes. (Nessler 

 jars about 20 mm. in diameter and 280 mm. long are 

 preferable for this purpose, but the shorter form of 100 

 cc. Nessler tubes may be used). A series of twelve stand- 

 ards is made by diluting the stock mixture to the 100 cc. 

 mark as follows : 



Number of cubic centimeters of stock 

 mixture, to be made up to ioo cc. 

 Silicia standard. with distilled water. 



20 2.0 



30 3-o 



40 4-o 



50 5.o 



75 7-5 



IOO IO. o 



125 12.5 



150 15.0 



175 175 



200 20.0 



250 25.0 



300 30.0 



When the turbidites are low, the samples may be corn- 

 pared with the standards by holding both over sharp 

 black lines on white paper, and looking through the tubes 

 lengthwrse. They may be compared also by examining 

 them sidewise towards the light or over black lines, thus 

 largely eliminating the effect of color. The higher stand- 

 ards are opaque when viewed lengthwise, and must be ex- 

 amined sidewise towards the light, or over black lines at 

 an angle with the light. During the comparison of the 



