POLISHING METAL SECTIONS
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equal to zero. On the other hand, the failure of Moissan' and of R u f f ' to obtain any evidence for the existence of free NH, a t very low temperatures is strong evidence to the contrary. Furthermore, it has been shown by the writer3 that the alkali and alkaline-earth metals exist in mercury, even in dilute solution, not in the form of single atoms, but in that of compounds of the general formula hlHg,', containing only one atom of the amalgamated metal to the molecule. Ammonium amalgam behaves analogously to these, and we are justified in concluding that it also is a solution of a compound of the general formula hf.Hg,,, in mercury. T h e solution of the coinpound (NH,)Hg,, is not stable, however, except at very low temperatures. I t decomposes slowly at oD,more rapidly a t room temperature, into mercury, ammonia, and hydrogen. T h e gases become entangled in the mass and give rise to the characteristic inflation, which, however, is not a property of the original compound, but merely an accidental phenonietion accompanying its decomposition. T R B A N AILL. , February, 1907.
A NEW APPARATUS FOR POLISHING METAL SECTIONS. BY K . W. ZIMMERSCHIED. Received March 21,1907.
I n the teaching of metallography, the question of proper apparatus with which to provide students for polishing the sections has sometimes presented itself. Where one is working alone, a machine such as is found on t h e market, with horizontal spindle carrying regularly four polishing wheels of different fineness answers very well, but where a number of persons must be provided for, this arrangement is quite unsatisfactory, since the finer wheels are too much exposed to contamination by material that will scratch the final surface of the metal, and the care requisite to avoid this contamination is exercised by a relatively small percentage of students. Again, t h e outlay necessary to provide a sufficient number of such machines for a large class, and then to supply each with means for starting and stopping without interference with the work of other operators, is considerable. T h e delivery of water to the surface of a wheel which revolves in a vertical plane also offers decided disadvantages, appreciated by all who have used the system, and the difficulty of taking care of the water thrown by such a wheel is still more annoying. Having a number of men to take care of in a limited space, the author has devised the apparatus herein described, which overcomes a number of these difficulties, and which he hopes may help to solve t h e problem for others similarly situated,
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Compt. rend. 133, 713 ; 715 ; 771 (1901). Moissan obtained t h e reactions : NH,Cl ,NHBLi.: = L i Cl. NH, (NH, H ) ; 2NH& H,S = Li,S t 2NH, H, ; etc., in liquid ammonia, at temperatures approaching - xw0. Ber. 34, 2604. Ruff worked a t - g s 0 , and a t pressures up to 60 atmospheres. Am. Ch. J., 36,124 (1905); Ibid., 37, 506 (1907).
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