Triply ditkrential cross sections in coplanar geometry for the ionization of alomic hydrogen by electron impact have been measured and calculated at incident electron energies of 54.4, 27.2, 17.6 and 15.6 eV. The theory is the same as that used successfirlly at higher energies arid is shown to repioduce nearly all features of the CTOS sections in this low-energy regime as well. Moreovcr, the physical origin of peaks and dips in the triply differential cross sections has been explained. We conclude that kinematical effects, exdiarige effects, correlation and the coherence between electron-electron and electron-proton scattering are all important at t h e e enerejes.

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TWELVE years ago (Trans., 1882,41, SS), the compounds CeF,H,O and 3KF,2CeF4,2H,0 were described by me, and it was shown that, when carefully heated, they lose their water, and then, if heated more strongly, they evolve fluorine, that is, a gas which smells of hypochlorous acid and decomposes potassium iodide with liberation of iodine." About the same time (1882, Abstr., S), it was stated by me that similar compounds of quadrivalent lead had been prepared, but that I was unable to obtain them in a pure state.In the year lW5, Nikoljukin (Journ. Russ. Chem. SOC., 1885) announced the existence of a double salt of ammonium chloride with lead in w-hi2h the lead is present as tetrachloride.This salt was subsequently investigated (1887) in this laboratory by Friedrich, and lead tetrachloride mas obtained from it (1890, Abstr., 699). The salt has the formula 2NH,CI,PbC14 (Friedrich :Abstr., 1893, ii, 415 ; compare Classen and Zahorski, Abstr., 1893, ii, 464, and Wells, ibid., ii, 523). Another interesting compound of quadrivalent lead, the tetracetate, has been studied recently by Hutchinson and Pollard (Tra.ns., 1893, 63, 1136).tThe work on the fluorides of quadrivalent lead referred to above w a s taken up from time to time, but my laboratory not being arranged for this kind of work, the deleterious effects of hFdrofluoric acid often compelled me to postpone it. I beg now t o lay before the * Moissan regards i t as possible that some higher fluorides may decompose by heat to lower fluorides with liberation of fluorine, but he makes no reference to my experiment, by which this was proved. MendelBeff, however, in his w-ell-known Princ+les of Chemistry, says : " It is eridently fluorine." -f Last year N r . Husnik, one of my advanced students, was engaged in i nvestigating Jacquelain's salt, Pb02(C4H303), ; it was ibund that the true formula is Pb(C2H302),, and that it can be easily obtained by following Jacquelaiu's direction. When Mr. Husna began to prepare other salts of tetratomic lead from it, the paper of Hutchinson andiPollard was published, and I regarded it as an act of courtesy due to these English chemists t o discontinue the research. * This is brown when the fusion takes place a t a low temperature, and yellow at higher temperatures. I n the latter case, the hard yellow melt obtained will contain not only potassium plumbate, but also large quantities of potassium peronide, which dissolves in hydrofluoric acid with liberation cjf oxygen. A t the same time an insolubls crystalline substance is obtained which, judging from analogy, is most probably pofassium jZiiorox.vperp?um€~ate ; the latter is being investigated.

IN continuing my researches on the chemistry of the '' rare earth "

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