122 TERMINOLOGY. 



struraent. This weight will show the capacity of the instrument; 

 no body of greater weight being capable of having its specific grav- 

 ity ascertained by it. Let us suppose the capacity of the Araeometer 

 to be known ; we proceed as follows to learn the specific gravity of 

 a mineral. A fragment of it is placed in the upper pan C, and 

 weights are added, until the mark b coincides with the surface of 

 the water. The amount of weight thus added, is subtracted from 

 the balance weight; the remainder is the weight of the mineral in 

 air. The mineral is now transferred to the bucket E. But as all 

 bodies weigh less in water than air, it will be requisite to add more 

 weight to the pan C, in order to bring the mark b to its appropriate 

 level. The amount added, in this last case, will be exactly the 

 weight of a quantity of water equal in bulk to the mineral. We 

 are thus brought acquainted with the absolute weights of equal 

 bulks of water and of the mineral, and the ratio of these weights is 

 the ratio of their specific gravities. 



The Hydrostatic Balance is the most accurate, and is therefore to 

 be chosen for nice inquiries, such as determining the gravity of very 

 minute specimens, and where it is the object to fix the limits of the 

 range in the specific gravities of a new species. But for the com- 

 mon purpose of finding the specific gravity of minerals, in order to 

 find out their names by the aid of the characteristic, the Araeometer 

 will be found every way sufficient, and generally preferable, on ac- 

 count of the greater expedition with which it can be used. Besides, 

 it is much cheaper and more portable. 



The minerals of which we are taking the specific gravity should 

 be perfectly pure. The greatest care must therefore be exercised, 

 in removing, as much as possible, whatever foreign substances ad- 

 here to them. And further, we must avoid employing such speci- 

 mens as contain vacuities. In order to get rid of these, the miner- 

 als must be broken down into fragments, until we can select such 

 as appear perfectly continuous, even when viewed by the micro- 

 scope. Compound varieties are more liable to contain cavities than 

 simple minerals; for this reason, the composition must be overcome, 

 at least so far that it cannot have any more influence upon the accu- 

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

 ced in size, since this might lead into an opposite error, in supposing 

 those minerals lighter than water, which swim upon it, when redu- 

 ced to an impalpable powder. 



