V. On osmotic pressures derived from vapour-pressure measurements: Aqueous solutions of cane sugar and methyl glucoside

Abstract

Since our previous communications on cane sugar (‘Roy. Soc. Proc.,’ A, vol. 1906) and on calcium ferrocyanide (‘Phil. Trans.,’ A, vol. 209) we have endeavoured to improve the apparatus and method therein described. In these endeavours, two objects, besides that of determining osmotic pressures, have been in view: (l) to find out what are the best conditions for making the method both accurate and of general applicability; (2) to obtain data for the construction of suitable apparatus for the determination of the absolute vapour density of liquids in air or other gas. As far as accuracy is concerned, the close agreement between osmotic pressures observed directly, and those here calculated from the vapour pressures, shows that, for solutions at 0°C. the method is susceptible of considerable precision. The experiments at 30°C., however, are on the whole not quite so concordant; this is probably because the quantity of water vapour in the air stream is some six times the amount carried at 0°C., so that small variations in conditions which would be without effect at the lower temperature, now make themselves apparent. This, indeed, is shown clearly in the magnitude of the correction to be applied for the expansion of the air between the solution and water vessels (see p. 316). In the early stages of the work (at 0°C.) our results indicated that the successful application of the method depends largely on the control of the experimental conditions; accordingly we made as many changes as possible, but as each experiment takes several days to complete, a long time is required to determine the effect of any one change. The departure of Mr. Hartley for the front prevented a further investigation of the question but we think that no new changes will alter substantially the results now presented.