the mechanism of water sorption is somewhat different. It may under some standard conditions, the rate of loss of poison from well consist of the transference of water into the plant cells by a composition, which between these so-called leaching rate diffusion and osmosis, slow processes which could account for determinations, is kept either in the sea or in indoor laboratory the great length of time necessary for equilibrium to become tanks with changes of esternal solution. However, considerable established. As with the fibres, the shape of the regain curve discussion has arisen regarding the accuracy of the method. suggests a change of mechanism at 70-80% R.H. Some data will therefor be analysed which bear upon this particular aspect of leaching rate work.
Various modifications of the above general technique have Acknowledgrncnts
The work described.foqns part of the research programme of been described (see subsequent references), but the one used in the Department of Scienufic and Industrial Research and Fire this work is that given by Harris6 (for some further details, see Offices Committee Joint Fire Research Organisation and is Barnes','~~).
The primary object of both the latter workers published by permission of the Director. We are greatly was to examine a large variety of compositions, rather than to indebted to Prof. T. P. Hilditch, F.R.S., of Liverpool University, determine the leaching rates of a small number of compositions for advice in certain matters and for having our esperimental ivith extreme accuracy. The sources of errors in such determaterials analysed in his laboratory. The materials themselves minations may be summarized as follol\*s : were provided by the Ministry of Food, through Lieutenant-I. PUbJtblg-(i) Errors in sampling the composition; (ii) &lone1 T. L. Bazley-Green, formerly of the Fire Prevention errors in painting technique; (iii) errors due to \*ariable Branch, to whom our thanks are due. conditions during the drying of the composition. 2 . Esposrcre-(i) Errors due to different positions of exposure ; (iii) Errors due to the effect of variable amounts of slime or fouling accumulating on the slides during the exposure periods. the leaching apparatus (particularly risk of damage) ; (ii) errors during leaching process, temperature changes, PH changes, bubbling rate changes ; (iii) errors in analysis of leachate for poison content, errors of manipulation, errors inherent in the actual technique. It is necessary to describe in some detail the technique employed, for the subsequent statistical analysis to be of use to General conditions of working Department Of Engineering and (ii) errors in transferring slides [particularly risk of damage) ;