least 40 years and I can see no advantage to be gained by returning to em3, which was never really adopted by a majority of chemists. As Mr. Ronneberg admits, the abbreviation ml is better anyway. Yet it is remarkable that the error in the attempt to make the liter exactly one cubic decimeter is only 27 parts in one million. I wish I could work as accurately. WILLIAM T. HALL S ~ P A TROAD ~ T Roc~esr~n, MASSACHUSETT~
~nkmonics To the Editor: May I be allowed to add the following two mnemonics to your steadily increasing list? The essence of Vorlander's Rule concerning the directing influences of snbstitnents in the benzene ring is that a snbstituent containing a Double bond directs Meta and that a substituent which contains only Single bonds or which consists of a Single atom directs Ortho and Para. "DAM SOP" therefore summarizes the Rule. In order to remember that the sixth, eighth, and tenth members of the fatty acid series are caproic, caprylic, and capric acids and in this order, it is noted that the second vowels of the three words are o, y, and i-memorized as "Oh my eye." UNIVERSITY OP MELBOURNE
Atomic Weight Units
To the Editor: In connection with Fred Fordemwalt's question "Why hasn't the unit of atomic weights been given a name!" CHEM.EDUC.,17, 545, (Nov., 1940)], the following quotations from "Chemistry--An Elementary Text-book," by MORGAN AND LYMAN, (The MacmiUan Co., New York City, 1913, pp. 106-7) may be of interest :
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"According t o Dalton's theory, combining weights are simply the weights of the atoms of the different elements; i. 8.. the atomic weights. We should remember, however, that the idea of atomic weights is part of a theory only. . That each element has a reacting weight is a fact, however, that will stand as long as there is any science a t all. "In determining the relative weights of the atoms of different elements, the weight of the hydrogen atom was taken as 1. The weight of the hydrogen atom is often called a minocrith. . . "On the basis of oxygen 16, speaking accurately, we should have to define a microcrith as of the weight of the oxygen atom and say that the hydrogen atom weighs 1.0075 microcriths."
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The present writer, however, believes it is confusing to call a ratio of two weights a unit. To talk in terms of a microcrith gives one the feeling that he is talking about a definite unit. LAWRENCE E. WUKINS UNNERSITY OP SOUTRBRN CALIFORNIA Los ANGELES, CALIPORNIA
