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I. I~T~ODUCTIOZ~. I N the first part of this communication T a method of studying the rate of solution of air by water was described, and some results were given, which showed that, when the water was kept thoroughly mixed and the waterair surface unbroken, the phenomenon took place in accordance with the general equation dw z w in which a represents the initial rate of solution, and bw the rate of escape of the gas from the water, b being a constant depending on the conditions of the experiment. The method of experimenting consisted in enclosing a large bubble of air, of known volume, in a narrow tube containing de-aerated water, and allowing the bubble to pass

In the Philosophical Transactions, Vol. 170, pp. 257—274, 1879, one of us has described, in conjunction with Mr. A. K. Huntington, the first use of dry gelatine films, sensitised with silver bromide, for photographing ultra-violet spectra; and the application of the almost continuous spectrum emitted by the metals iron, nickel, and copper to the purpose of examining the ultra-violet absorption spectra of organic compounds. These researches, up to the present* time, have been prosecuted under considerable disadvantages, owing to the impossibility of describing accurately either absorption or emission spectra, by reason of the data for calculating wave-lengths being unfortunately insufficient. The object of this work is to give an exact description of the photographed spectra of some sixteen elementary substances, and to place on record the wave-lengths of so large a number of well-defined metallic lines, together with such other measurements of spectra, that subsequent workers may experience no difficulty in constructing interpolation curves capable of yielding fairly accurate numbers representing wave-lengths. The first measurements of rays of high refrangibility made by means of photography were the determinations of the wave-lengths of the lines of cadmium by M. Mascart (‘Annales de I'École Normale,' vol. iv., 1867). He made use of a Nobert’s grating, a goniometer, and a photographic eye-piece. In addition to the splendid map of the ultra-violet portion of the solar spectrum given us by M. Cornu, we have wave-lengths most carefully calculated for a series of lines in the spectra of the metals cadmium, magnesium, aluminium, and zinc (‘ Annales de l’École Normale,' vol. ix., 1880; and ‘Archives des Sciences Physiques et Naturelles de Genéve,' (3), ii., pp. 119-126). Messrs. Liveing and Dewar, with an improved goniometer and a Rutherfurd grating, have estimated the wave-lengths of the lines of carbon by a modification of Mascart’s method (Proc. Roy. Society, vol. xxxii., 1882). As each line must be cussed independently of the rest of the spectrum and photographed on a different plate, and as the relative positions of the lines on the photographs are varied by very slight alterations in focus and by the removal and replacement of the plates, we have been led to apprehend that there are grave objections to this method of manipulation. The process, moreover, appears to be a lengthy one. In accordance with these views, which are the result of long experience, we have preferred to employ a method similar to that of Cornu.* As much of each spectrum as possible is photographed on one plate, and together with this a series of ideal lines or plain reflections of the slit, each corresponding to a measured angular deviation, from which a scale of wave-lengths may be calculated.

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THE estuary of the River Liffey, or rather that portion of i t included within the Dublin Harbour, affords, from the point of view of sewage pollution, one of the most interesting and instructive objects of study to be found around the British Isles ; and as the Authors have recently had occasion to make a careful examination of it, from both an engineering and a chemical point of view, a record of the results with a short explanation of them may prove of practical value. The estuary, Fig. 1 , is confined on the south side by the South Wall, and on the northeast by the North Wall, which is about 9,400 feet in length; of this about 6,000 feet, from its commencement at the Dollymount shore, is constructed to a height of 6 feet or 7 feet above high water, the remaining 3,400 feet being carried up to half-tide level only. Consequently, when the tide is at half-ebb the whole of the remaining effluent water in the estuary is concentrated and discharged through the entrance at Poolbeg, where the channel is only 1,000 feet in width, and a very rapid current, varying between 275 feet and 300 feet per minute, is set up. Unassisted by any dredging, this current has entirely swept away the bar which formerly obstructed the entrance to the harbour. After the harbour has been emptied of tidal water, very little of the water so discharged, excepting at low neaps, returns into it with the ensuing flood-tide ; the harbour being then filled, not with previously discharged water, which has passed out to a point about a mile beyond its mouth, but with clean seawater coming up with the flood-tide from the direction of Eingstown.