52 THE AMERICAN MONTHLY [Feb 



edge of the exact weight of the suspended matter. Con- 

 sequently indefiuite terms have beea discarded and more 

 accurate systems of numerical expression have come into 

 use. 



Of the various methods that have been sujrgestedjOnly 

 three have commended themselves to {)ractical use ; name- 

 ly, the Wire Method, the Diaphauorneler, and the use of 

 Standards of Comparison. The iirat is a rapid method for 

 obtaining approximate results, and serves well as a local 

 standard and for the general purposes of field work, but 

 it is not applicable to laboratory use. The second re- 

 quirs a somewhat elaborate ai)paratus and conditions of 

 observation not readily obtainable in all laboratories. 

 The third method is simple and quite satisfactory for gen- 

 eral use. Standards of kaolin have been used for this 

 purpose, but they are open to the objections that they 

 cannot be readily du[)licated and do not keep well when 

 employed in connection with the color standard. These 

 difficulties appear to be overcome by the use of finely pow- 

 dered diatomaceous earth instead of kaolin. This earth 

 may be obtained in almost any part of the country, and. 

 is prepared for use in the following manner : 



The diatomaceous earth, as pure as may be obtained, is 

 washed with water to free it from any soluble salts, and 

 ignited to remove the organic matter. The perfectly 

 white earth thus obtained is tlieu treated with warm hy- 

 drochloric acid (1 : 1), after wliich it is washed with dis- 

 tilled water by successive decantations until free from the 

 acid. The material, now composed of practically pure 

 diatom frustules is then ground in an agate mortar to an 

 impalpable powder and again shaken with distilled water. 

 Any coarse particles that may be present are easily sepa- 

 rated from the fine particles by sedimf^ntation and decan- 

 tation. The fine material is then allowed to settle, sepa- 

 rated from the water, dried at 100° C, cooled in a desic- 

 cator, and placed in a stoppered bottle. The material 



