Mr Ventress's Stereometer. 143 



Art. XXX. — Description of a Stereometer. By James 

 Alexander Ventress, Esq. Communicated by the 

 Author. 



The bulk of a known weight of solid, of irregular figure, is 

 generally obtained by immersion in water. In the common 

 weighing bottle for ascertaining the specific gravity of solids 

 in the state of powder, a known weight of the powder is in- 

 troduced into a phial, and the interstices filled up with water, 

 the difference between the quantity of water required to fill 

 the phial when empty, and after the powder is introduced, 

 being evidently the bulk of the powder. But if the powder 

 dissolve in water or the liquid employed, we obtain by this 

 means not the space which the substance occupies in the solid 

 state, but that which it occupies when in solution. The dif- 

 ference is considerable and irregular. 



The stereometer which I propose is not liable to error 

 from this source. In the coniometer of Captain Say and Mr 

 Leslie errors of this kind are also avoided, although in a way 

 totally different. 



To illustrate the principle of the instrument proposed, let 

 us suppose a quantity of any powder or saline body, which 

 we wish to examine, to be introduced into the common weigh- 

 ing bottle, and a phial full of water adroitly inverted over it, 

 so that the system was air-tight. The water in the superior 

 phial would descend, and fill up the interstices of the powder, 

 while the displaced air would ascend and occupy the upper 

 part. It is evident that the quantity of air in the upper phial 

 will represent the interstices of the powder, or the difference 

 between the bulk of the powder and the capacity of the weigh- 

 ing bottle, which is known. In the weighing bottle we learn 

 the bulk of the solid by marking the quantity of water neces- 

 sary to fill up the interstices ; in the stereometer proposed, by 

 measuring the air which previously occupied these interstices. 

 The following construction is found to be particularly con- 

 venient for that purpose. 



As in the illustration, two vessels, A and B, are employed, 

 (See Plate II. Fig. 3,) A being analogous to the weighing 



I 



