SCIENCE MARCHES ON "
SIR,—In Dr. Sherwood Taylor's notice of my book, Science Marches On, which appeared in your columns on September z5th, he complains that I am mistaken in identifying modern sal ammoniac with the sal ammoniacum of Pliny and Dioscorides. In point of fact, I do not mention sal ammoniac anywhere in my book, but merely state that ammonia was known to Pliny and Dioscorides, which is in accord with the best authoritative opinion.
Again, Dr. Taylor charges me with inaccuracy in giving the value of a millionth of a gram of radium as " about six- pence." I do not know how he has checked this rough estimate (which is for metallic radium), but if his own figure of " a penny " is derived from the present commercial price of radium chloride, it is still nearly 5o per cent. wrong.
I am indebted to Dr. Taylor for pointing out two insignifi- cant errors, one of which was due to an artist's slip, but I do feel that the rest of his remarks, often inaccurate as the above examples show, concern trivialities which fail to justify the disparaging tone of his review. The object of the book is to convey " the sort of advance in man's thinking which is now under way in the world of science," as is stated on page 323, and eminent men of science have been very appreciative of the way in which this has been done.—Yours, &c., 29 lames Street, London, W. I. WALTER SHEPHERD.
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