Polarization qfPlatina Electrodes. 469 



trate of silver, a mixed precipitate of chloride, bromide, and iodide of 

 silver is obtained ; the two first are dissolved by ammonia, whilst the 

 iodide remains insoluble ; the weight of it then determines that of the 

 iodine contained in the sample. 



The new method proposed by M. Lassaigne is founded on the great 

 insolubility of iodide of palladium, which is such that :f oc/oo o*^ °^ ^^ 

 alkaline iodide is not only detected by the nitrate or chloride of pal- 

 ladium, but the resulting iodide is precipitated in brown flocks, in 

 twelve or sixteen hours, and may be collected on a weighed filter. 

 The weight of the dried iodide indicates that of the alkaline iodide ; 

 it is composed, according to M. Lassaigne, of two atoms of iodine and 

 one of palladium. 



[In English weights of 1 eq. iodine 126+54, 1 eq. palladium 

 = 180, therefore 180 of the iodide of palladium = 126 iodine.] 



The method of proceeding is that of treating a certain quantity of 

 kelp with repeated portions of water, mixing the solutions, saturating 

 with weak nitric acid and adding nitrate or chloride of palladium, 

 when the liquor becomes immediately brown by the iodide of pal- 

 ladium formed. When the quantity is considerable it is readily de- 

 posited, but when small, as already noticed, it requires from twelve 

 to sixteen hours to deposit; this reagent is capable of detecting 

 quantities of iodine which are not shown by starch, chloride of pla- 

 tina, or nitrate of silver. — /owrw. de Chem. Med., August, 1838. 



POLARIZATION OF PLATINA ELECTRODES. 

 M. Matteucci has communicated the following results of his ex- 

 periments to the Academie Royale des Sciences. 



1 . Plates of platina, used as electrodes in water, become covered 

 with traces of oxygen and hydrogen, which they retain for a certain 

 time. 



2. Plunged into oxygen or hydrogen, platina plates acquire and 

 preserve for some time a coating of these gases. 



3. Plates thus treated and immersed in distilled water develope an 

 electric current moving from the hydrogen to the oxygen plate. 



M. Schcenbein also communicated the following remarks on the 

 same subject. 



A tube curved in the shape of the letter U and filled with pure 

 hydrochloric acid having a plate of platina plunged into each leg, 

 was placed in communication with the ends of a voltaic battery. 

 After a few minutes the electrodes were removed and fresh plates 

 immersed; a feeble secondary current, detectableby a very delicate gal- 

 vanometer, was discovered ; this passes in a direction opposed to that 

 of the battery. A similar result occurs when dilute sulphuric acid 

 is substituted for hydrochloric acid. From these circumstances M. 

 Schcenbein concludes that electrolytes are capable of becoming vol- 

 taically paralysed under the influence of a current. He also stated 

 that he does not agree with M. Becquerel in his explanation of the 

 phaenomena of polarized plates, because he has observed electrodes 

 to assume the property of polarization when they have conveyed a 

 current of too feeble intensity to decompose iodide of potassium. For 



