44 



[Prof. J. Dewar and A. Scott. 



[Feb. 15, 



stopped down to an aperture corresponding to the size of the mirror,, 

 the reflected image is contrasted with that seen directly, and if the- 

 definition is unchanged, the mirror is marked K.O., with a writing 

 diamond, and returned to the maker ; if the object appears distorted, 

 its unfitness for use is similarly notified. A small fee is charged for 

 the examination. 



III. "On the Atomic Weight of Manganese." By JAMES 

 Dewar, M.A., F.R.S., Jacksonian Professor, Cambridge, 

 and Alexander Scott, M.A. Received February 9, 1883. 



Our attention has been directed for some time to a new determina- 

 tion of the atomic weight of manganese. This communication gives 

 a succinct account of the results of the preliminary stages of such 

 an inquiry, and although the further progress of the investigation 

 may reveal some errors, still we feel convinced the final numbers can 

 in no way differ materially from the present values, and therefore 

 further delay in publication is unnecessary. 



The atomic weight of manganese has been determined by many 

 chemists,* but the resulting values vary considerably according to the 

 special method selected. The results of the different investigators 

 may be divided into two classes — those giving approximately 55 

 as the number, and those making it about 54. To the former 

 class belong Turner, Berzelins, and Dumas, all of whom use the 

 same method, viz., the determination of the silver chloride yielded 

 by a weighed amount of chloride of manganese. Turner also made 

 determinations from the analysis of the carbonate, and from the con- 

 version of the monoxide into sulphate. Yon Hauer used the same 

 method as that employed by him in the determination of the atomic 

 weight of cadmium, viz., the reduction of manganous sulphate to 

 sulphide by ignition in a current of sulphuretted hydrogen. It is- 

 probable that this method is not very trustworthy, as, according* 

 to Schneider, the sulphide may be contaminated by oxysulphide. 

 Schneider and Rawack belong to the second class of observers, the 

 former employing the oxalate, and from its analysis calculating the 

 atomic weight by deducting the weight of water and carbon dioxide 

 obtained. Rawack, whose experiments were conducted in Schneider's 

 laboratory, weighed the water obtained by reducing manganoso- 

 manganic oxide to manganous oxide. 



One objection to the analysis of the chloride is that it may contain 

 besides manganous chloride varying proportions of manganic salt. 



* Berzelins, "Lehrbuch," 5 Ed., 3, 1224. Dumas, "Ann. Chem. Pharm.," 113, 

 25, 1860. Hauer, " Wien. Acad.," xxv, 124. Rawack and Schneider, " Pogg 

 Ann.," 107, 603. 



