1900] MICROSCOPICAL JOURNAL. 53 



thus prepared for use consists of particles of silica that 

 are about 1 micron (.001 mm.) in diameter, and that are 

 fairly uniform in size. 



A stock mixture is made by adding 0.5 gram of the 

 prepared diatomaceous earth to 500 cc. of distilled water. 

 This is put in a liter bottle in order to give room for vio- 

 lent agitation before measured portions of it are with- 

 drawn. The mixture contains 1 gram of pure silica per 

 liter, or 1,000 parts per million. This is absolutely un- 

 acted upon, either by distilled water or by the platinum- 

 cobalt color standard, and when well shaken will always 

 give the same turbidity. 



For waters of low turbidities, standards are made up in 

 gallon bottles. The stock mixture above described is call- 

 ed a standard of 1,000. Five standards are made from 

 this by dilution, as follows : 



Number of cubic centimeters of stock 

 mixture, to be made up to three liters 

 Silica standard. with distilled water. 



o, 5, io, 15, 20 o, 15, 30, 45, 60 



Nearly all of the natural waters of New Engtand fall 

 below the standard of twenty. These comparatively clear 

 waters may be examined by putting them in gallon bot- 

 tles like those holding the standards, and viewing them 

 together toward the light It is well to have under the 

 bottles a black surface that extends backward for about 

 1 foot, rising at an angle of about 30°. This enables the 

 observer to distinguish the particles in suspension, to 

 judge the amOunt of light that they cut out, and to esti- 

 mate the turbidities even though the particles vary 

 greatly in size. 



Silicia standard. Present system. 



o None. 



1— 2 Very Slight. 



2— 5 Slight. 

 5— 20 Distinct. 



above 20 Decided. 



