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MULTIPLE DECK STEAM

PIPE OVENS FOR DRYING PURPOSES

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WATER found in nature is never pure. It contains something not water which forms bubbles on the sides of the containing vessel when the temperature is raised rather rapidly as well as encrusting solids which separate later. This observation must have been made very early; certainly soon after glass vessels were used for heating. was not, however, until Cavendish had shown how to manipulate gases that the study of such a subject was possible.

It

The discovery of Henry in 1803 that "under equal circumstances of temperature water takes up, in all cases, the same volume of condensed gas as of gas under ordinary pressure," was followed immediately by Dalton's further generalisation that from a mixture of gases an amount of each gas is dissolved dependent upon the specific solubility of the gas and its partial pressure in the mixture. Dalton gave the following example from his own work-Atmospheric air, consisting of 79 parts of azotic gas and 21 parts of oxygenous gas per cent.

Water absorbs 1/64 of 79/100 azotic gas=1*234. Water absorbs 1/27 of 21/100 oxygenous

gas=0'778.

Showing that he had a fair but not very exact knowledge of the solubility of the gases of air in water. He also sought to establish a simple atomic packing of the gas and water atoms of his new atomic theory using a shot piling analogy and diagrams in three dimensional space.

For half-a-century work on the gases in water seems to have been confined to sporadic observations. The methods of investigation available were neither exact or unsuitable for the work which was attempted. This was mostly undertaken afloat in connection with various expeditions sent out by England and France to explore the surface and depths of the ocean. By this means a certain amount of useful information was obtained and published, but an experimental study of the solubility of gases in water and other liquids and of the kinetics of interaction was needed if the subject was really to be advanced. Bunsen, who had brought the methods of gas manipulation to much greater perfection than Cavendish had left them, investigated in 1855 the solubility of many gases in water and other liquids.

It had been generally realised that water freed from air and then exposed to or even shaken with air required a rather long period to reach saturation, although the amount of air taken up was very small, but the first suggestion as to the course of this process seems to have been made by Dittmar. In his report on the physical and chemical work of the great Challenger Expedition, published in 1884, he wrote: "Supposing a certain portion of the ocean were separated from the rest, and after some had been deprived of its *Delivered at Finsbury Technical College, October 14, 1920.

gaseous contents, exposed to the air at a constant temperature of t°, the three gases would stream into the water at a steadily diminishing rate until absorptiometric equilibrium was established, i.e., a point reached when, for instance, the number of molecules of oxygen dissolved in a given small time would be exactly compensated by the same number of previously dissolved oxygen molecules returned to the atmosphere." The Challenger reports were issued in a vast number of folio volumes, and I suppose few chemists or physicists could afford to buy them. Besides, the investigation of the velocity of chemical reaction was not at that time an attractive subject to chemists.

All work on the rate of absorption or escape of gas from water, from whatever point of view undertaken, leads to the same conclusion, namely, that the reaction between

a

liquid and a gas followed the same course as that between other heterogeneous phases which have been investigated. We may consider the case of a gas over perfectly still water. The surface rapidly becomes saturated; in fact, there is good reason for considering the surface interchange to be instantaneous, but the downward diffusion of the gas in the liquid is slow and proceeds with greater slowness as the depths are reached owing to the decreasing pressure due to the retention of gas in the upper layers of the liquid. Nevertheless, a continual entry at the surface and procession downwards occurs until the whole of the liquid is saturated with the gas. This case can be treated by means of the application of Fourier's theorem, which is known as Fick's law of diffusion, It has been investigated in the United States by Black and Earle Phelps, but as Adeney and Becker have very recently pointed out, it is most difficult to realise in practice and seldom or never occurs in any actual case needing investigation. The diffusion is extremely slow, and unlike most cases of diffusion of concentrated solution, is not aided appreciably by differences of density, as these seem to be inappreciable. I have not been able on looking at the available data to decide whether water saturated with air is denser or lighter than pure

water.

The very slow diffusion can, however, be supplemented by mechanical agitation, in which case the water can be considered as divided into two layers, an upper.one where the concentration gradient falls off from saturation at the surface to some lower concentration which is uniform throughout the lower part of the liquid. With increasing rate of agitation the layer of varying concentration becomes thinner and thinner, that is the liquid is uniformly mixed.

The rate of solution of a solid in a liquid was first investigated by Bogusky for marble dissolving in acids and later by Noyes and Whitney and Nernst and Brunner for various not very soluble substances, e.g., benzoic acid. It is an analogous case to that which we are considering, and all workers on the absorption of gases by water, or the escaped gases from water, have found an expression based on Newton's law of cooling to hold good. The rate of absorption dx k (C—x)

dt

where C represents the concentration of full saturation and x the concentration at any moment t,