28 SOME PRINCIPLES AND METHODS OF ROCK ANALYSIS, [bull. 176. 
tion one-half gram is a very convenient weight. In general, it may be 
made a rule not to use more than 2 grams for any portion which has to 
be fused with an alkali carbonate, as for sulphur, fluorine, and chlorine. 
For carbon dioxide the weight may rise to 5 grams, or even more, if 
the amount of this constituent is very small, without expenditure of 
any more time than is required by 1 gram, and with correspondingly 
greater approach to correctness in the result. For vanadium also a 
larger weight than 2 grams is usually demanded. 
II. SPECIFIC GRAVITY. 
BY SUSPENSION IN WATER. 
Ordinary method. — This determination, when required, is best made 
upon one or several fragments weighing up to 20 grams. They are 
held together by a fine platinum wire ready for suspension from the 
balance, and thus held are placed in a small beaker to soak over night 
in distilled water under the exhausted receiver of an air pump, side by 
side with a similar beaker of water. Boiling is, of course, a much less 
effective means of removing air than the air pump, and the boiling 
water may exert an undesirable solvent and abrading effect. In the 
morning the wire is attached to the balance arm, the rock fragments 
remaining immersed in the water; a thermometer is placed in the com- 
panion beaker of water, now likewise in the balance case, and the weight 
is at once taken. Both vessels of water having precisely the same 
temperature, it is quite unnecessary to wait for the water to assume 
that of the balance should it not already possess it. The fragments 
are now lifted out, without touching the vessel, and carefully transferred 
to a tared crucible or dish; the wire is removed and at once re weighed, 
with the precaution that it dips just as far into the water now as when 
weighted. Hereby a special weighing of the wire out of water is 
avoided. The sample may now be dried on the water bath and then 
at 110° C. for some hours to certainly expel all absorbed water, and 
weighed after prolonged cooling in the desiccator. It is better to 
ascertain the weight of the dry rock after soaking in water than before, 
in order to avoid the error due to possible breaking off of a few grains 
between the two weighings. Should the density of the rock in air-dry 
condition be required, it may be left exposed to the air for a long 
period after drying and before weighing; 1 but the difference will only 
1 In view of the uncertainty as to what constitutes hygroscopic water (see p. 35), this course is per- 
haps more to be commended than the former, and seems imperative for certain zeolitic rocks. In 
such cases it is best to weigh the fragments before putting to soak, and afterwards to collect on a Gooch 
crucible the grains which may have fallen off in the water. Should no crucible of this kind be avail- 
able, a paper filter may unhesitatingly be used and incinerated with the powder, owing to the small 
amount of which the error due to loss of even all its water during ignition is quite negligible. 
