Royal Society. 53 



which becomes positive; but with steel the reverse obtains. It 

 would appear, however, that with the same pairs of metallic 

 discs, the direction of the electric current is determined by the 

 nature of the acid employed: thus nitrous acid, acting upon 

 zinc, copper, or iron, gives rise to a current in a direction oppo- 

 site to the current which is produced by the sulphuric, nitric, or 

 muriatic acids. Variations in the temperature of the metals will also 

 occasion diversities in the results, not hitherto satisfactorily ex- 

 plained on any theory. From one experiment the author is led to 

 infer, that an acid is capable of combining with a pure metal, with- 

 out the latter being previously reduced to the state of an oxide. 



May I't. — A paper was read, entitled " On the Composition of 

 the Chloride of Barium;" by Edward Turner, M.D. Professor of 

 Chemistry in the University of London. Communicated by the 

 Rev. Dr. Lardner, F.R.S. 



The frequent employment of chloride of barium in delicate che- 

 mical investigations, renders an exact knowledge of its composition 

 peculiarly desirable ; and this has become a more important object 

 of inquiry since it has been made by Dr. Thomson the basis of his 

 calculation of the chemical equivalents of sulphuric acid, and of 

 thirteen metals and. their protoxides. He has deduced from his ex- 

 periments with the chloride of barium, the number 36 as the equi- 

 valent of chlorine, 70 as that of barium, and 78 as that of baryta; 

 whence the equivalent of the chloride of barium would be 106 : and 

 accordingly, on mixing this quantity of the chloride with 88 parts 

 of sulphate of potash, each being previously dissolved in separate 

 portions of distilled water, he finds a complete double decomposi- 

 tion has taken pl^ce ; the resulting sulphate of baryta reduced to 

 dryness, weighing 118 parts; and the muriate of potash yielding 

 76 parts of chloride of potassium. Hence he infers that 40 is the 

 equivalent number for sulphuric acid, and 48 that for potash. Ber- 

 zelius, however, maintained that this experiment, as well as the 

 deductions from it, are not exact. Dr. Thomson having, in con- 

 sequence of Berzelius's objections, repeated his experiments, still 

 asserts their accuracy. The author of the present paper investi- 

 gated the subject with the greatest care, employing materials in a 

 state of perfect purity, and obtained results which coincided with 

 those of BerzcUus. He details the precautions he took for insuring 

 the conditions of perfect purity in the substances with which his 

 experiments were made, and to the neglect of which he traces 

 some of the errors which he imputes to Dr. Thomson's analysis. 

 But there exists also a more radical cause of error in the method 

 employed by that chemist; for Dr. Turner finds, that when solu- 

 tions of muriate of baryta and of sulphate of potash arc mixed to- 

 gether, a small portion of the latter salt adheres tenaciously to the 

 Buipliatc of baryta, which is precipitated, and thus escapes decom- 

 position. By employing didercnt processes, the author avoids this 

 source of fallacy. First, from the chloride of barium, previously 

 dissolved ill water, he throws down sulphate of baryta by adding 

 sulphuric acid; and secondly, lie eflccts a precipitation from a 



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