Mr. J. Brown on the Molybdate of Lead. 253 



talogue stars, only sixteen fall out of this category in 1850; 

 viz. 



Urs. Min. 2320, 4070, 4150, 4165, 6281, 6320, 6999, 7184, 

 Octans, 71, 2878, 5936, 5959, 6793, 7020, 7713, 8072; 

 a satisfactory result, as these polar segments = :j^yth=^^f7-ths 

 of the spherical surface. 



With my constants, seven of those sixteen (2320, 4070, 

 4150, 5936, 6281, 6320, 7713) have all their R.A. coefficients 

 positive, the others have some negative. Indeed, near the 

 pole the annual change is so great, as to render greater con- 

 stants to include a dozen of these sixteen needless. 



The saving in the above 8362 stars permits an additional 

 column, although five fig. logs, are required, giving a result as 

 far as O^-Ol or 0"-01. 



App. R.A. = mean R.A. at epoch + yearly precession + 

 proper motion + ephemeral quantity. 



+ Ee+F/+G^ + HA+ stellar quantity — lO"". 

 App. N.P.D. = similar quantity — 10', 



which 10' might be already included in the mean place, as in 

 the planetary tables. The ephemeral quantity (depending on 

 the day of the year) is the same in R.A. time-seconds, or 

 N.P.D. space-sccojids. 



Possibly these hints may be useful before reprinting the 

 British Association Catalogue. 



S. M. Drach. 



London, Sept. 2, 1847. 



XLII. On the Molybdate of Lead. By Mr. John Brown*. 



MOLYBDATE of lead was first analysed by Klaprothf, 

 who proceeded in the following manner : — 

 J 00 grains of the mineral finely pounded were treated with 

 dilute hydrochloric acid, and the whole of the silica was thus 

 separated. Upon cooling, the greater part of the chloride of 

 lead was deposited in fine crystals. The clear supernatant 

 liquor was then drawn off, and when sufficiently concentrated 

 the remaining chloride of lead was deposited. The whole of 

 the chloride was then carefully collected together, dried and 

 weighed. Its weight was 74*5 grs. From this the quantity 

 of oxide of lead was ascertained, which was 64*42 grs. Every 

 100 grains of molybdate of lead contain therefore 64*42 grs. 

 of oxide of lead. When the solution had thus been freed from 

 lead, it was concentrated by evaporation. Nitric acid was 



* Read before the Philosophical Society of Glasgow, April 28, 1847, and 

 communicated by Dr. R. D. Thomson. 



t Beitrage zur chemiscken Kenntniss der Minerat/corper, vol. i. 265. 



