Chemical Society. 343 



expressing the specific gravity by 2-77 ' , and then deducting 360 from 

 the result, gives of course the saccharometer indication or gravity ; 

 thus ii^. — 360 = 36. Many works, held in high estimation, by way 

 of facilitating this operation, have adopted the use of the factor 2-78, 

 but this must of necessity involve error without materially shortening 

 the calculation; some parties have gone so far as to state that 2-7 

 is a sufficient approximation ; this, however, with the gravity taken 

 as an illustration, will give an error of 11-4 pounds in excess, or 47-4 

 instead of 36. The rule adopted by the author for converting real 

 specific gravities, or hydrometer indications, into saccharometer gra- 

 vities, is as follows : — From the specific gravity observed, expressed 

 in terms of distilled water as unity, deduct 1 , and then multiply the 

 result by 3 60 ; the product is the equivalent saccharometer indication ; 

 thus for specific gravity MOO; MOO-l-OOOx 360 = 36 of the sac- 

 charometer. The saccharometgr gravity again is calculated from the 

 real specific gravity, by the converse of this operation ; divide the 

 saccharometer indication by 360 and then add 1 ; thus ^ -t- 1000 

 = 1-100. 



Dec. 7. — The following communications were read : — 



" On a new Class of Cacodyl Compounds containing Platinum," 

 by Professor Bunsen * of Marburg. (See Memoirs, Vol. I., Art. 10.). 



" On the Preparation of Chromic Acid," by Robt. Warington, Esq. 



In the number of L'Institut for 9th July 1840, under the head 

 of " Proceedings of the Imperial Academy of Sciences of St. Pe- 

 tersburgh," a notice is given " On an easy process for preparing 

 chromic acid, and the manner in which it behaves with sulphuric 

 acid," by M. I. Fritzsche. The author pours concentrated sulphuric 

 acid with care into a hot and saturated solution of the bichromate of 

 potash, and obtains a voluminous scarlet crystalline precipitate, 

 which is separated and dried, first by heat, then in a vacuum. This 

 is the chromic acid, which must be washed with a small quantity of 

 cold water to remove the mother liquors and sulphuric acid which 

 may still adhere to it. As to the compound of sulphuric acid and 

 chromic acids described by M. Gay-Lussac in the Annales de 

 Chimie et de Physique, vol. xvi. p. 102, the author says " he has 

 never been able to make it, and is very much disposed to doubt its 

 existence." On repeating this process, I found that the chromic 

 acid does not fall alone, but is contaminated by admixture with a 

 considerable quantity of a white saline substance, which on exami- 

 nation proved to be the bisulphate of potash, and which, on account 

 of the great solubility of botli these substances as precipitated, there 

 is great difficulty in separating. The modification of this process, 

 which I have found to give chromic acid in a crystalline form and 

 nearly in a state of purity, is to take 100 measures of a cold satu- 

 rated solution of the bichromate of potash (prepared by boiling and 

 then allowing the solution to cool and deposit the excess of the salt), 

 and add to this from 120 to 150 measures of concentrated sulphuric 

 acid ; the latter should be free from sulphate of lead, as otherwise it 



[* A translation of Prof. Bunscn's Researches on the Cacodyl Series 

 will appear in the forthcoming Part of Taylor's Scientific Memoirs.— En.] 



