1. Absorption of carbohydrate probe molecules from ligated loops of rat small intestine was studied. Absorption was determined by measuring recovery of molecules in the urine, corrected for incomplete recovery after intravenous injection, and was examined for correlation with several parameters of molecular dimension. 2. Absorption depended on molecular volume rather than relative molecular mass, molecular radius or molecular area. 3. Molecules with a molecular volume below 225 X 10(-3) nm3 were absorbed to a greater extent than larger molecules, and absorption was affected critically by molecular volume, small changes in volume producing considerable variation in absorption. 4. Absorption of larger molecules was not affected by changes in volume within the range 362 X 10(-3)-1128 X 10(-3) nm3. 5. These findings support the concept that there are at least two aqueous diffusion pathways across the intestinal mucosa. small molecules diffusing through a small channel of finite dimension, compatible with a transcellular aqueous pore, whilst large molecules diffuse through a less frequent pathway of considerably larger dimensions.

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The pressure dependance of Sommelet-Hauser and concurrent Stevens' rearrangements of some 'onium salts has been investigated. The ionic reaction has been shown to be very susceptible to reduced pressure, whereas competing radical reactions are relatively unaffected. ments at low pressures are discussed.PREVIOUS work 1~ showed that quaternary hydroxides undergo elimination reactions at a vastly increased rate (relative to the competing substitution reactions) at low pressures, and this was attributed either to the increased basicity of the unsolvated anion or to removal of water from the transition state. It was thought likely that other types of reaction, in which the ratedetermining step was dependent upon the basicity of the anion, would be affected in the same way.Accordingly, diphenylmethyltrimethylarnmonium hydroxide (I; R1 = Ph, R2 = Me) was pyrolysed atThe reasons for the enhanced rate of Sommelet-Hauser rearrange-160"/760 mm., and the products were compared with those from the same decomposition at 60"/0-005 mm. Earlier work had shown that o-benzyl-NN-dimethylbenzylamine (11; R = Ph) was the principal rearrangement product of the high-temperature pyrolysis, together with some NN-dimethyl-a-methylbenzylamine 3r4 (111; R1 = Me, R2 = H), but the evidence for the structure of the latter was equivocal. I n our hands, the high-temperature pyrolysis of (I; R1 = Ph, R2 = Me) gave a mixture (see Table) of NN-dimethyldiphenylmethylamine (111 ; R1 = R2 = H), a-benzyl-NN-dimethylbenzylamine (11; R = Ph), NN-dimethyl-(3,fbdiphenylethylamine (IV; R1 = H, R2 = Ph) and a new base p-benzyl-NN-dimethylbenzylamine (V; R = Ph) (Scheme 1). Despite a rigorous search, no trace of (111; R1 = Me, R2 = H %' ) was detected. The base (V; R = Ph) may be formed by intermolecular attack of the ylide (VI) upon the para-position of (I; R1 = Ph, R2 = Me), and subsequent loss of diphenylmethanol. It is not possible for (V; R = Ph)

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