32 SOME PRINCIPLES AND METHODS OF ROCK ANALYSIS, [bull. 176. 
required by the work in hand. Either the mechanical arrangement is 
complicated or cumbersome, requiring more power or space than is 
usually at disposal or causing too much noise, or thorough cleansing 
is difficult and troublesome, or there is likelihood of contamination 
from oil or grease or lack of facility for the removal of all powder from 
the mortar. These last defects are especially prominent in those forms 
in which the mortar is fixed in its setting. 
All rock samples have therefore been reduced to powder by hand, 
involving a great expenditure of time and labor. Ordinarily an ex- 
tremely fine state of division is unnecessary, except in the case of those 
portions in which alkalies and ferrous iron are to be estimated or 
where soluble constituents are to be removed by acids, etc. , and in such 
cases the final grinding can be done at the balance table on a small 
portion slightly in excess of the quantity to be weighed off. 
The process of sifting through fine cloth, the German "Beuteln," is 
not one always to be commended, because of the time required and, 
more especial!} 7 , because of the certainty of contamination by cloth 
fiber, which in the ferrous-iron portion might affect the result. Still 
less should metal sieves be used. 
WEIGHT OF GROUND SAMPLE. 
The sample when ground should weigh not less than 10 grams, and 
preferably 20 in case it should be necessary to repeat or advisable to 
employ unusually large portions for certain determinations, notably 
carbonic acid. Rock analysis has in this respect an advantage over 
mineral analysis, since material is almost always available in ample 
quantity and any desired number of separate portions may be used, 
whereas with a mineral the analyst is frequently compelled to deter- 
mine many or all constituents in a single, often very small, portion of 
the powder. This course often involves delay and the employment of 
more complicated methods of separation than are usually necessary in 
rock analysis. 
IV. WATER— HYGROSCOPIC, ZEOLITIC, CRYSTAL. 
Importance of employing air-dry powder for analysis. — The time- 
honored custom of drying a powdered specimen before bottling and 
weighing has long seemed to the writer one that has no sound basis 
in reason. Its object is of course plain, namely, that of securing a 
uniform hygroscopic condition as a basis for convenient comparison of 
analytical results, since some rocks contain more hygroscopic moisture 
than others. Nothing, however, is more certain than that by the time 
the substance is weighed it has reabsorbed a certain amount of mois- 
ture, small, indeed, in most cases, but very appreciable in others; and 
