Improvement in Say's Instrument/or taking Specific Gravities. 203 



logarithms, but at the same time show their use in analytical 

 investigations. 

 July 1834. J. R. Young. 



XXXI. Description of an Improvement in the Construction 

 of Say's Instrument for measuring Specific Gravities, By 

 W. H. M.* 

 A DESCRIPTION of this instrument as originally con- 

 ££• structed, and which might with more propriety be called 

 an instrument for measuring volumes, is given by its 

 inventor, M. Say, a French officer of engineers, in ^Q-, 

 one of the volumes of the Annates de Chi?niefor 1797 

 (xxiii, i.), and also by Mr. Faraday in his work on 

 Chemical Manipulation. The annexed figure repre- 

 sents the instrument in its improved form. ABare two 

 glass tubes, of 0*2 inch internal diameter, one 34 and 

 the other 35 inches long, placed close together, and 

 having their lower ends cemented into an iron cap, 

 into the lower end of which an iron stop-cock is ^ 

 screwed. The upper end of the longer tube is ce- 

 mented into the bottom of a cup B, having its rim 



ground truly plane, the capacity of which is a little in D 



more than half that of the tube. The tubes are L. 



fixed parallel to a graduated scale, carrying a sliding 

 vernier D, provided with an index formed of two 

 slips of brass, one before and the other behind the 

 tubes, having their lower edges in a plane perpen- 

 dicular to the scale. E is a piece of plate-glass, having 

 its lower surface greased, and large enough to close 

 the mouth of the cup B. The instrument may either 

 be fixed permanently in a vertical position against 

 a wall, like a barometer, or else may have a broad foot with 

 three screws, by which it may be rendered vertical for use. 



The substance to be examined, which may be any solid 

 liquid or powder that is not volatile, is placed in a small cup, 

 which goes into the cup B. The stop-cock at A is closed, and 

 mercury is poured through a small funnel into the shorter 

 tube till it rises to a mark on the longer tube at P ; the 

 mouth of the cup B is then closed, so as to be air-tight, by 

 the plate of glass E. The stop-cock must now be opened, 

 and the mercury permitted to escape in a small stream till 

 its surface in the longer tube stands about 15 inches higher 

 than in the shorter tube, when the stop-cock is to be closed. 



* Communicated by the Author. 

 2 D2 



