JOURNAL OF CHEMICAL EDUCATION
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In connection with the article by Mathews' on the construction of electric flask heaters, your readers may be interested to know of a somewhat similar method of construction which was described by the writer in 1949.2 The chief points of difference are that the elements are wound on asbestos string which is in turn wound onto the inverted flask and plastered with asbestos lagging pulp, and secondly, in the place of cans ordinary flower pots are used. These have the advantage of being nonconducting and because of their tapered bases they can be fitted into the ordinary ring supports. The method of construction is not limited to single been used for a bath for flask hut has eieht Soxhlet units. A limited number of reprints of the paper cited are available for any who care to write for them.
To the Editor: While teaching sophomore quantitative analysis during the past several years, I have noticed a peculiarity about the commonly used 50-ml. Exax burets. I have been wondering if others among your readers might either confirm or deny my observations. Each sear I have the students calibrate their burets. MATHEWS, FREDERICK, J. CAEM. EDUC., 28,258 (1951). BUTLAND. M. S.. J . New Zealand Inst. Chem.. 13, 53 (1949).
Student calibrations, especially at the beginning of the year, are notoriously unreliable. Nevertheless, a striking regularity is revealed from some hundreds of calibrations. The over-all corrections to the buret,s are always positive (correction to be added to the nominal volume to obtain the true volume). The huge bulk of the burets show over-all corrections of +0.05 to +0.10 ml. with a few showing corrections of +0.15 ml. or a little more. I have checked a fair number myself, especially if - a student has obtained a nezative correction. I have never been able to substantiate such a result. Never in mv memory have I found an Exax buret with a negativeoorrection. This result surprises me considerably since normal manufacturing error should be distributed more or less symmetrically about the true value. Incidentally, the calibrations were made in the usual way by weighing the delivered water. Since room temperature usually runs between 20" and 25°C. and the water used usually is a degree or so below room temperahre, a fixed factor, 1.003, was used to convert the observed weights to true volumes. This considers both the absolute -density of water and an air bnovancv ~~~~"~~ ~< correction. ~ ~ ~ ~ The above results have been communicated to the ~ i ~ , ~l~~~ , l ~ company and they have kindly expressed interest and an intention to check a t t,he mannfacturing ~
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To the Editor: In the September, 1951, issue of the JOURNAL OF CHEMICAL EDUCATION, Mr. Milton G. Wolf of James Madison High School, Brooklyn, New York, wrote a letter t o the Editor in which he made known that upon attempting to dissolve selenium in olive oil, aided by heat, he obtained later a solid material. He also stated that he was uncertain about the composition of this solid, and would welcome comment. It seems quite possible that what he has obtained
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DECEMBER, 1951
is the trans state of the oleic triglyceride instead of the normal predominating cis state. Oleic acid is the principal fatty acid in olive oil. It is well known that treated under proper conditions with certain oxides of nitrogen, oleic acid (either as a free acid or in the esterified state) ~vililltransform itself from the cis isomer (which is liquid a t room temperature) to the trans isomer which is a solid. It may be that, at higher temperatures, selenium may also catalyze this transformation. I t seems again possible that the selenium may have combined chemically a t the double bond in permanent fashion (thus destroying the double bond as such) and thereby creating something structurally akin to stearic acid, or a cross-link arrangement leading to a polymer structure. In view of the fact that Mr. Wolf states that the reaction was carried out a t high temperature (probably about 600'-700°F.) each of the above alternatives may have occurred, together with other possibilities such as air oxidation, etc., all of which would be likely to lead to solid endproducts from oleic esters. JOEIN COLLOIDAL PRODUCTS CORPORATION CALIFORN~A SANFRANCI~CO.
To the Editor: West (J. CKEM.EDUC., 28, 437 (Aug., 1951)) states, in connection with the use of molecular weights in teaching quantitative analysis, that ' I . . . this is one of the few places were it seems safe to assume prior knowledge."
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The experience of the present miter is entirely different. I have found that students come into quantitative analysis with very hazy, incomplete, or even decidedly incorrect concepts of molecular weight. One of the most common misconceptions is the mistaken idea that molecular weights can be obtained by adding the atomic weights indicated by a chemical formula-any kind of chemical formula. On the contrary, this method can be used only if the formula is a molecular formula. Adding the atomic weights represented by an empirical formula does not give a molecular weight. The only formula known for any salt or other electrovalent compound is the empirical formula, and molecular formulas are not known for such compounds. It is impossible to measure the molecular weight or to calculate the molecular formula of a salt or other electrovalent compound by any means whatsoever, because such compounds consist of ions, not molecules. Indeed, the concepts of "molecule" and "molecular weight" of electrovalent compounds is completely meaningless. No rational definition of either molecule or molecular weight can possibly have any significance when applied to electrovalent compounds. The illogical and incorrect conce~tof "molecular weight of s h s " is held by so many &dents in quantitative analysis that I am led to the suspicion that it is not entirely the students' fault. It is quite possible that they have learned these illogical ideas from some of the commonly used textbooks. J. C. HACKNEY EASTCHICAGO, INDI.~NA
