OWING to absence from the laboratory during the summer, I have only recently been able to make experiments on the effect of silica on acids, using silica prepared as described by Prof. Mukerjee in his letter to NATURE of August 29. The silicon tetrachloride was added to water in a silica dish and the mixture dialysed until the specific resistance rose to 120,000 ohms. It was then dried at the air temperature without the use of any desiccating agent. I have been unable, however, to detect in this product the slightest adsorbing power for hydrochloric acid. In his letter of April 4, Prof. Mukerjee gives figures from which I infer that he found that silica could take up more than I per cent, by weight of hydrochloric acid. I used one gram of silica and 100 c.c. of N/500 acid and should easily have detected the adsorption of less than 0.0004 gram hydrochloric acid (i.e. 0.04 per cent, by weight) by the conductance and pH measurements used. But not the slightest sign of removal of hydrochloric acid from solution was obtained.