A study has been made of the desorption and absorption of gases by liquid drops during the period of their impact on various types of surfaces, following previous work on absorption during formation and during flight of drops. On impinging on a surface, drops form spray, and ‘residual liquid’ which is not detached from the surface and ultimately drains away. The amount of desorption in the spray does not vary greatly with either the height of fall of the drops or the type of surface. In the residual liquid, poor desorption is obtained with ‘dry’ surfaces, i.e. well‐drained surfaces with an average liquid depth of less than 0·1 mm., and relatively good desorption with ‘wet’ surfaces. Desorption in the residual liquid increases to a moderate extent with the velocity of drops before impact, but appears to be little affected by the nature or the contour of the surfaces tested. The process of absorption or desorption is interpreted in terms of the dynamics of the impinging drop. For desorption measurements, water drops containing carbon dioxide were used; as far as could be judged by a limited number of experiments, absorption of oxygen by drops of sodium dithionite or sulphite solution appeared to be affected by factors similar to those found in desorption.

A field test for determining small amounts of hydrogen cyanide in industrial atmospheres is based on formation of a Prussian blue colour on test paper impregnated with ferrous sulphate and sodium hydroxide. The test is specific for hydrogen cyanide. The behaviour of a number of possibly interfering gases has been investigated. The test is sensitive to slightly less than 1 p.p.m. of hydrogen cyanide in air and has an error of f 10 to 20 per cent.Test papers properly prepared and stored retain their activity for a t least 10 months.The blue stains obtained are permanent.TWO field tests for the detection and approximate estimation of hydrogen cyanide in air are described in "Methods for the detection of toxic gases in industry-Leaflet No. 2" (Department of Scientific and Industrial Research).l One of these, the benzidinecopper acetate test, has been widely used, but it has certain drawbacks, e g . , difficulty in controlling the critical moisture content of the test paper, instability of the reagents, of the prepared test paper and of the stains on the test paper after treatment with hydrogen cyanide, and lack of specificity. The alternative Congo red-silver nitrate test shares some of these drawbacks.Tests based on formation of a Prussian blue colour from hydrogen cyanide, which would have the advantages of specificity and permanence of pigment, have usually been regarded as insufficiently sensitive and unsuitable for use as field tests.2 Gettler and Gold-baum3 determined the amount of hydrogen cyanide in solutions by aerating at 90" C and passing the gas through test paper previously impregnated with ferrous sulphate and sodium hydroxide to form blue stains of various intensities. These authors claimed that the test papers would retain their usefulness for several weeks if stored in a cool dark place. Our tests showed, however, that test papers prepared as described were sometimes unsatisfactory, and that anyway the papers could not be relied upon to retain their full activity after a few hours. We finally succeeded in establishing the conditions necessary for a test based on formation of a Prussian blue colour, which was free from the various drawbacks already mentioned. EXPERIMENTAL TEST PAPERS-The success of the method depends largely on the correct preparation and storage of the test papers. These are prepared by immersing filter-paper in ferrous sulphate solution, drying, immersing in sodium hydroxide solution and again drying.Reactions within the test fi@ev-The two reagents that are incorporated in the test paper can react with each other and with atmospheric carbon dioxide or oxygen. Ferrous hydroxide, which is initially formed by the action of sodium hydroxide on the ferrous sulphate, can be preserved for long periods in spite of a reputation for instability. Shipko and Douglas4 have shown that ferrous hydroxide in contact with saline solution containing either excess of hydroxide or of ferrous ions is stable for periods up to 6 months if oxygen is rigorously excluded. When the dried paper already impregna...