Mr Ven tress's Sttreometer. 145 



fit into each other by screwing. To avoid the slight com- 

 pression of air in A, which it would be difficult to avoid in 

 screwing, a minute hole might be drilled into B at p, into 

 which a small screw should be passed after the connection 

 between A and B is completed. * 



Should a portion of the substance of which we wish to as- 

 certain the specific weight be dissolved by the water, which 

 occurs in nearly all saline bodies, the liquid would be slightly 

 expanded, or contracted from that cause, and consequently 

 the air above would be proportionally condensed or rarified. 

 When solution takes place the liquid more generally contracts, 

 as the density of a solution is generally greater than the mean 

 density of its constituents. The air above is therefore in most 

 cases of solution slightly rarified. 



But this is easily corrected by separating the parts A and 

 B under water, and then sinking B till the surface of the wa- 

 ter in the stem coincides with the surface of the water with- 

 out. In this way we obtain the true specific gravity of the 

 substance in the solid state. 



It is proper always to employ water which has been recent- 

 ly boiled, and which contains little air, to fill the upper vessel, 

 as many salts, when dissolving, appear to disengage the air 

 which water holds absorbed, which, mixing with the air from 

 the interstices, would derange the calculation. It has been 

 found that air passing up so rapidly through the boiled wa- 

 ter, as it does in the present case, is not sensibly absorbed, so 

 that no error of an opposite kind is committed by employing 

 water previously deprived of most of its air. 



Art. XXXI. — Notice of some new observations hy Mr 

 Brooke on the SulphatO'tri-carhonate of Lead. By Wil- 

 liam Haidinger, Esq. F. R. S. E. 



Mr Brooke has communicated to me, in a letter of the 14th 

 of May, some very important observations on the optical 

 structure of certain varieties of the sulphato-tri-carbonate of 



* Stereometers, with such modifications, may be had handsomely exe- 

 cuted, from John Dunn, optician, 25, Thistle Street, Edinburgh. 

 VOL. VII. NO. I. JULY 182T. K 



