308 TERMINOLOGY. . 209- 



specific gravity of such bodies as do not possess any great 

 absolute weight. 



The pin, which supports the uppermost cup of the araeo- 

 meter, destined for the reception of the weights and of 

 the body to be weighed, must possess a certain diameter, 

 since it acts not only as a supporter, but also in the 

 capacity of a real weight, according to the depth to which 

 it is immersed in the water. This diameter, however, 

 must remain within certain limits, if the instrument shall 

 not lose its niceness. 



The hydrostatic balance will therefore be more eligible for 

 more accurate inquiries, either for obtaining a greater num- 

 ber of decimal figures, or for determining the gravity of a very 

 small specimen : hence it must always be employed, if our ob- 

 ject is to fix the limits of the range in the specific gravities 

 of a natural-historical species, for the sake of the determina- 

 tion of its varieties ; and this has been done in the species 

 contained in the subsequent Characteristic. For the com- 

 mon use of determining the specific gravity of minerals, in 

 order to find out their denominations by the assistance of 

 this Characteristic, the araeometer will be found both suf- 

 ficient and preferable, because in this case we may ac- 

 quiesce in most cases in the first decimal figure of the spe- 

 cific gravity. The instrument is besides recommendable, 

 on account of its being cheap and portable. The size of 

 the specimens, the specific gravity of which may be taken 

 by help of the araeometer, cannot exceed certain limits, 

 determined on one side by the absolute weight it will bear, 

 till it be immersed to the sign marked upon the pin, on the 

 other by the niceness of the instrument itself. 



In taking the specific gravity, we must likewise observe 

 the degree of temperature. The changes of temperature 

 render it necessary to determine the normal weight, or that 

 which is required for depressing the araeometer to a cer- 

 tain point, at every experiment, in the same way as it is 

 necessary in the hydrostatic balance, which likewise, previ- 

 ous to every experiment, must be brought into equilibrium. 



The minerals, of which we intend to take the specific 



