Volts. Copper replaced by zinc = +0.960 Observed values of K2. +0.920 to +0.960 -0.340 to -0.385 Whilst with zinc, lead, copper, and cadmium, the observed values of K, in no case differ very widely from those equivalent to the differences in heat of formation, those observed with silver show large differences, indicating as before that silver exhibits a high negative value for its thermo-voltaic constant in each case, viz., -0.5 to −0·6 in contact with dilute sulphuric acid, and near to -04 in contact with ammoniacal fluids, this latter value being close to those found previously for silver in contact with neutral solutions of its sulphate, nitrate, and acetate (loc. cit.). On the whole, except when an oxidisable metal is used exhibiting a high negative value for its thermo-voltaic constant, the E.M.F. of a cell containing an aëration plate and an oxidisable metal always falls short, and sometimes largely short, of that equivalent to the chemical changes going on therein even under the most favourable conditions when generating only an infinitesimal current, the deficiency being still more marked when the current density is not so minute in other words, the modus operandi of cells of this class is such as necessarily to render a large fraction of the energy nonadjuvant so far as current is concerned. Just the same remarks apply, as far as our experiments have gone, to cells in which the oxidisable substance is in solution, an extreme case of which is exhibited by cells set up with a solution of sulphurous acid and a submerged platinum foil plate, opposed to an aëration plate of platinum sponge on the surface of dilute sulphuric acid. Such cells give an E.M.F. (when generating only extremely small currents) of from 0.2 to 03 volt, whilst the heat of oxidation of sulphurous acid solution, SO,aq,0, is 63634 gram-degrees, according to Julius Thomsen, corresponding with 1.368 volt, or upwards of a volt more than that actually produced. Analogous diminutions in E.M. F. are brought about in many other cases, to extents depending not only on the nature of the aëration plate but also on that of the oxidisable fluid. A large part of the depreciation in this case is due to the fact that sulphurous acid solution and platinum constitute an oxidisable portion of a cell behaving as magnesium and aluminium do in cells where they replace zinc, i.e., giving a much smaller E.M.F. than that due to the heat corresponding with the chemical change: thus, if a cell be set up with zinc or dilute sulphuric acid opposed to platinum in sulphuric chromic acid solution, and the zinc and sulphuric acid be then replaced by platinum and sulphurous acid solution, the E.M.F. falls by an amount greater by 0-45 to 0.5 volt than that corresponding with the differences in heat evolution between Zn,O,SO.aq and SO2aq,O (viz., 106090 - 63634 = 42456 gram-degrees 0-913 volt): and similarly with other oxidising fluids. Solutions of alkaline sulphites behave similarly. = Effect of Substituting Oxygen for Air. In order to see if any material improvement in the E.M.F. of aëration cells could be effected by substituting tolerably pure oxygen for atmospheric air, we carried out a number of observations with plates under a bell-jar supplied with purified oxygen from a reservoir by means of tubes passing through a cork in the narrow mouth. Readings were first taken for a few days with ordinary air in the jar; oxygen was then admitted and passed through till gradually all air was displaced, and after a day or two when the readings had become constant another series of readings for some days was taken. The oxygen was then displaced by air and another series taken, and so on alternately several times. The following average values were ultimately obtained showing a small, though decided, increment in E.M.F. when atmospheric air was replaced by oxygen. Aeration Plates in Contact with Oxidisable Atmospheres. Some analogous experiments were made with aëration plates in contact with an oxidisable atmosphere (hydrogen or coal-gas), and an electrolytic fluid united by means of a siphon with an external vessel containing an oxidising solution (alkaline permanganate, sulphuric acid containing chromic acid, nitric acid, &c.) in which a plate of platinum foil was immersed. The readings thus obtained were nothing like as concordant as those above described (probably from the difficulty of excluding air completely), showing a tendency to rise continually. The following readings were obtained after several days when the rise had either ceased or greatly slackened in most cases; little difference was observed whether pure hydrogen or coal-gas was used. A. Cells set up with 715Na,0,100H,O in contact with the aëration plates, opposed to platinum foil immersed in a solution of the same strength shaken up with powdered potassium permanganate to saturation. B. Cells set up with 10H2SO,,100H2O in contact with the aëration plates opposed to platinum immersed in the same liquid after agitation with chromic anhydride to saturation. In making these observations currents were used, the density of which in no case exceeded 0.02 micro-ampère per square centimetre of aëration plate surface. Spongy platinum and palladium obviously are far more effective as regards the E.M.F. set up than the other plates used; the chemical action taking place may be regarded as the decomposition of alkaline permanganate into hydrated manganese dioxide, caustic potash, and oxygen (or of chromic anhydride and sulphuric acid into chromium sulphate, water, and oxygen), and the combination of hydrogen with the oxygen thus set free; according to Thomsen's values, the heat developed would accordingly be per 16 grams oxygen evolved Hence, even with the most effective plates, the E.M.F. actually generated falls distinctly short of that corresponding with the heat of chemical change. On making the current passing larger by diminishing the external resistance, the E.M. F. always fell rapidly; so that in order to obtain a current capable of producing any considerable amount of electrolytic decomposition in a voltameter, it was practically impossible to have an acting E.M.F. as high as 1 volt, even with tolerably large platinum sponge plates. Much the same result was obtained on opposing to one another two platinum sponge aëration plates, one in an atmosphere of hydrogen or coal-gas, the other in contact with air; in no case could any current capable of depositing a few milligrams of silver per day be obtained with an E.M.F. as great as 1 volt; i.e., a total depreciation of upwards of 0.5 volt was occasioned, or more than one-third of the energy due to the chemical change, viz., oxidation of hydrogen to water, representing 68360 gram-degrees, or 1470 volt. The economical production of currents by the direct oxidation of combustible gases, therefore, does not seem at present to be a problem likely to be readily solved. The Society then adjourned over Ascension Day to Thursday, May 17th. Presents, May 3, 1888. Observations and Reports. Barbados:-Report upon the Rainfall of Barbados, and upon its Influence upon the Sugar Crops. 1847-74. Folio. Barbados 1874. Calcutta-Meteorological Office. The Meteorological Office, London. The Office. Cape Town:-Meteorological Commission. Reports. 1879, 1881-83. Folio. Cape Town 1880-84. : The Meteorological Office, London. Edinburgh: Hamburg-Deutsche Seewarte. 4to. Hamburg 1880. India :-Archæological Survey of India. Report. Vol. XXIII. Geological Survey of India. Records. Vols. I-XXIII. 8vo. The Survey. Vol. XXI. Part I. The Survey. Observations, &c. (continued). New York:-Geological Survey of the State of New York. Paleontology. Vol. V. Part 1. Vol. VI. 4to. Albany 1885-87. The Survey. Nice:-Observatoire. Souvenir de la Conférence Géodésique, The Observatory. Session 1887. Obl. 4to. Trieste :-Osservatorio Marittimo. Rapporto Annuale. 1885. 4to. Trieste 1887. Upsala:-Expédition Suédoise au Spetsberg, 1882-83. Comptes Rendu. 8vo. Upsala 1884. The Meteorological Office, London. Berthelot (M.) Collection des anciens Alchimistes Grecs. Livr. I. The Author. Boltzmann (L.) Gustav Robert Kirchhoff: Festrede. 8vo. Leipzig 1888. The Author. The Author. Cassel (P.) Mischle Sindbad, Secundus - Syntipas. 8vo. Berlin 1888. Dawson (G. S.) Notes and Observations on the Kwakiool People of Vancouver Island. 4to. Montreal 1888. The Author. Foster (J.) Alumni Oxonienses: the Members of the University of Oxford, 1715-1886. Vol. I. 8vo. London 1888. The Author. Fourier (J. B. J.) Œuvres. Publiées par les soins de M. Gaston. The Author Jordan (J. B.) The Glycerine Barometer. 8vo. London 1881. The Author. Kölliker (A.), For. Mem. R.S. Ueber die Entstehung des Pigmentes Moukhtar Pasha (His Excellency) "The Garden of Moukhtar" [an |