310 Mr. Drach on the Use of Barometrical Formula 



of an excess of carbonate of soda ; dry the precipitate, and 

 place it in a coated green-glass tube, so disposed that the 

 whole can be heated to a dull red heat ; when red hot pass a 

 current of well-washed chlorine through the tube for a (evf 

 minutes : the lime will be converted into chloride of calcium, 

 but the magnesia remains unacted upon. When the whole is 

 cool, remove the mass from the tube and boil it for a minute 

 or two in water, filter the liquid and wash the insoluble por- 

 tion (which is magnesia) with water, and precipitate the lime 

 from the mixed liquors by carbonate of soda. The heat 

 should not exceed a dull red, as the mass is apt to become 

 vitrified at the part which touches the tube, and this renders 

 it difficult to remove the contents. 



To assay Gold, 



Take six grains of the gold to be assayed and place it in a 

 small crucible, with 15 grains of silver, and from 8 to 12 grains 

 of chloride of silver, according to the supposed impurity of 

 the gold ; lastly add 50 grains of common salt (chloride of so- 

 dium) reduced to a fine powder so as to prevent decrepitation ; 

 fuse the whole together for five minutes, and allow it to be- 

 come cold ; then take out the metallic button and beat it into 

 a thin plate, and subject it to the action of dilute nitric acid 

 as in the ordinary mode of parting. By this plan the tedious 

 process of cupellation is avoided, the baser metals being 

 wholly removed by the chlorine of the chloride of silver, and 

 their place supplied by pure silver. 



Old Barge-house, July 16, 1839. 



XLVII. On the Use of Barometrical Formulcefor determining 

 the Heights of Mountains. Bj/ S. M. Duach. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



IT has hitherto been generally supposed that our ignorance 

 of the law of the decrease of heat in the upper regions of 

 the atmosphere has prevented our obtaining an accurate for- 

 mula for the determination of heights by barometrical ob- 

 servations. I apprehend, however, that this has arisen from 

 an oversight in the analytical calculation ; which, if corrected, 

 would eliminate the efiect of the diminution of temperature 

 altogether. 



