JOURNAL OF CHEMICAL EDUCATION
668
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above correction in the calculational procedure, and also for pointing out a numerical error in the calculations I had originally made.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.
