I THANK you for your kindly notice of my little book in your issue of October 16 (vol. lxvi. p. 602), and I am quite content to leave your reviewer's remarks concerning its blemishes to the judgment of your readers with the one exception of that dealing with the precautions for preventing the contamination, of the carbonic acid gas with ammonia. If your reviewer will call to mind the fact that in the generating vessel there is a mixture with an alkaline reaction until the charge is exhausted, he will not consider it as so very astonishing that ammonia may pass into the gas holder. At all events, manufacturers of mineral waters have suffered too much in time past from the presence of gaseous impurities in the carbonic acid gas to permit them to allow the smallest trace of such impurities to contaminate the waters. The conditions of manufacture are such as not to warrant the expectation that either the alkali or the acid in the generator will suffice to hold back traces of either acid or alkaline gases.