. 209. PHYSICAL PROPERTIES OF MINERALS. 809 



gravity, must be perfectly pure. The greatest care there- 

 fore must be taken in removing as much as possible what- 

 ever heterogeneous substances may adhere to them, or at 

 least, if this should be impossible, not to neglect consi- 

 dering the influence of such an admixture upon the coi> 

 rectness of the results. Moreover, all the vacuities or 

 empty spaces within the specimens, must carefully be 

 opened. In order to get rid of these, the minerals ought 

 to be broken down, till, even by the assistance of a mi- 

 croscope, we can no longer detect a want of continuity in 

 the fragments. Compound varieties are more subject to 

 contain similar vacuities than simple minerals; for this 

 reason the composition must be removed, at least in so far 

 that it cannot have any more influence upon the accuracy 

 of our results. Yet the minerals must not be too much 

 reduced in size, since this might lead into an opposite er- 

 ror, in supposing those minerals lighter than water, which 

 swim upon it, when reduced to an impalpable powder. 



These precautions have been very often neglected in 

 taking many of those specific gravities quoted in mineralo- 

 gical works, and thus numberless erroneous and inaccurate 

 statements have been introduced, which render their em- 

 ployment at least uncertain, and on that account use- 

 less for Mineralogy. Another source of error, for which 

 many examples might be quoted, consists in the incorrect 

 determination of the natural-historical species to which 

 the specific gravities have been referred, and which have 

 passed from one work into another. 



A certain degree of attention is required, both in select- 

 ing the specimens and in the operation of ascertaining the 

 specific gravity. But from this it will appear that the re- 

 sults of the single experiments made upon specimens of 

 homogeneous minerals, coincide in a remarkable degree ; 

 and thus we may argue upon the great importance of the 

 application which this property will allow in the Natural 

 History of the Mineral Kingdom. 



