IN your issue of September 2 (p. 405) I find under the cover of a review of Cooke's “Chemical Physics” that Prof. Armstrong has been good enough to quote a passage from my “Lessons in Elementary Chemistry,” though without naming the source, concerning Avogadro's law, about which he asks the question, “Could anything be more misleading and inaccurate?” My friend appears to be no exception to the well-known rule as to critics failing to read the books they review, for a note on the same page (55) disposes of the “inaccuracy,” whilst the “misleading” statement is explained further on (p. 154). On the other hand, Dr. Armstrong has not followed the usual practice of critics, who, not being authors, escape from the danger of a retort courteous from those whom they find fault with; and hence I feel sure he will forgive me in saying that, whilst fully agreeing with him in the statement that a knowledge of mathematics is advisable for a chemist if he is to understand physics and physical methods, I still am bold enough to ask whether anything can be “more misleading and inaccurate” than the formula for reduction for temperature and pressure given in both editions of his “Organic Chemistry” under the description of Dumas's vapour-density method. And to add that no excuse can here be found of a correction given elsewhere, or of the fact that it may be desirable sometimes to state a case broadly to begin with and to define it more closely afterwards.