No. 4253 May 5. 1951 NATURE 725 graphy (Figs. 1-4), 20 µgm. protein being detectable. Protein degradation products do not cause a 'protein error' with the above indicators and, therefore, are not recorded on the chromatogram. Since neither bromothymol blue nor tetrabromophenolphthalein ethyl ester contains nitrogen, we are exploring the possibility that quantitative analysis of protein mixtures may be ach,ieved by nitrogen determinations of the spots. We should like to thank Dr. T. H. H. Quibell of the Chemistry Department, University of Manchester, for kindly preparing a specimen of tetrabromophenolphthalein ethyl ester.

In an attempt to obtain more accurate quantitative information on substitution reactions effected by free radicals and atoms, an experimental investigation has been carried out on the extent of substitution at the 0-, m-,

The light-induced chlorination of butane and 2-methylpropane by t-butyl hypochlorite has been been studied from 10 to 70°C in the gas phase. Light intensity, pressure, reactant concentration and temperature have been varied and the results have been rationalized in terms of chlorination proceeding by way of a t-butoxy radical chain carrier. The relative rates of hydrogen abstraction by the t-butoxy radical, kz RH+ (CH3)3CO* +R*+(CH3)3COH, have been obtained per H atom for the primary, secondary and tertiary positions of the two hydrocarbons studied.