other trunk mhere no moth stuff was placed. On removing these pencils some years later from their storage in the two trunks I was quite astonished that those which had been in the para-dichlorohenzene atmosphere (I guess it was this) were still in perfect condition, while those in the other trunk were now useless. What is the chemistry of this preservation, apart from the obvious fact that these pencils were not in the usual atmosphere?
To the Editor: Here is a problem for a chemist. It is common experience to discover that the erasers with which pencils come equipped often "go had," that is, they become hard and do not perform their function. The mechanism of what happens I do not fully understand, but suffice it to say that in these parts rubber suffers unduly from atmospheric effects. I believe that automobile tires show similar accumulative effects. Now the problem I wish to propose is this: Some years ago I acquired a stock of brand-new pencils, all equipped with good usable erasers. About half of these were a t one time put into a trunk (on the floor of the tmnk) wherein resided a heavy concentration of moth repellent (moth balls). The rest of these pencils were in an-
To the Editor: Recent work by Cunningham, Feay, and Rollier ( J . .4m. Chem. Soc., 76, 3361 (1954)), in which they describe the preparation of ThF4 and their lack of success in an attempt to prepare PrF4, suggests that an error in the calculatious of Perros, Munson, and Naeser (J. C u ~ ~ . E ~ u c . , 3 0 , 4(1953)) 02 should he called to your attention. The latter make the observation that the differences between the heat of formation of the trichloride and that of the triiodide are constant for several lanthanide and actinide cations. From this they reasonably assume that the difference between the
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heats of formation of trichlorides and trifluorides is a constant and estimate the heat of formation of PrFa. However, they next assume that the difference between the heats of formation of trichlorides and tetrafluorides is constant. This is incorrect since the cations being considered are not the same, but have undergone oxidation to the 4+ state. This means that the energy of oxidation, which differs considerably among the various cations, must he considered. The values given in their paper for the differences between the heats of formation of the trichloride and tetrafluoride of uranium, 230 kg.-eal./mole, and neptunium, 212 kg.-cal./mole, do not agree nearly so well as the constant differences quoted between the various trihalides. However, if one corrects these numbers by subtracting the difference between the heats of formation of the 3+ and 4+ ions in solution, namely 24 and 6 kg.-cal./mole in the respective cases (LATIMER,"Oxidation Potentials," 2nd ed., pp. 302-5) a new constant, 206 kg.-cal./mole, is obtained, the same in both cases. If one combines this constant with the value of the oxidation potential, EO = -2.86 v., given for the Pr+"Pr+' couple (LATIMER, p. 294), and makes the rest of the calculations as indicated by Perros, Muuson, and Naeser, one obtains the value A P = - 1 *24 kg.-cal./moie for the reaction PrFdc)
+ '/>Fdg)
=
PrFdc)
rather than the value AFo = -72 kg.-cal./mole. The results of Cunningham, Feay, and Rollier are consistent with this calculation.
I would like to thank Dr. Perros for a letter in which he agrees with the above correction in the calculational procedure, and also for pointing out a numerical error in the calculations I had originally made.
To the Editor: While soap bubbles blown by exhaled air are heavy and descend to the floor, those blown by hydrogen gas previously inhaled and stored in the lungs readily float up to the ceiling. When the air in the lungs is only partially displaced by hydrogen gas, bubbles of intermediate buoyancies are obtained. Some remain stationary in mid-air or move about sluggishly in the absence of air currents. When this hydrogen technique is concealed from the audience there is an element of surprise in the experiments. It is important that the hydrogen used for the experiments should be freed of all poisonous impurities such as arsine and others, if present, by storing the gas over alkaline permanganate solution in aspirator bottles.
