The expression ordinarily obtained for the electromotive force of a thermo-electric circuit is

The only assumption made in obtaining this equation was that the heat effects obey the two laws of thermodynamics.

It is thus seen that the same form of fundamental equation can be obtained in an entirely different way, by considering the energy of a thermo-electric current as part of the energy which resides upon every surface.

The last case is, in every respect, identical with the ordinary thermoelectric circuit between a solid and liquid. And if the results of Lippmann are accepted it has been shown that even the existence of neutral points can be explained.

If, as Maxwell says, there is for all bodies a coefficient of superficial energy, the energy of every thermo-electric circuit may be accounted for by changes in the potential energy of superficial tension.

XX.

ON THE SEPARATION OF TITANIUM AND ALUMINUM, WITH A NOTE ON THE SEPARATION OF TITANIUM AND IRON.

By F. A. Gooch.

Communicated May 13, 1885.

IN attempting to separate titanium and aluminum it is usual to resort to that method which depends upon the action of the slightly acid solution of the sulphates when submitted to prolonged boiling. The faultiness of this method, however, becomes apparent when solutions of aluminum and titanium are compared as to behavior under the conditions. To secure the complete precipitation of titanium the excess of sulphuric acid must be kept small, while to prevent the deposition of alumina the acid must be more than enough to dissolve the same amount freshly precipitated as hydrate, in the cold. Thus, upon boiling solutions containing the equivalent of 0.06 grm. of titanic oxide, 2.5 grm. of free sulphuric acid, beside alkaline sulphates, in 800 cm.3 of water, it was found that at the end of an hour 0.003 grm. of titanic oxide in one case, and 0.006 grm. in another experiment, had escaped precipitation; but that, when the solution of titanium had first been precipitated by ammonia and then made just acid by sulphuric acid, every trace of dissolved titanium was thrown out on boiling. The experiment, on the other hand, of boiling a gram of alum — roughly speaking 0.1 grm. of alumina -in 800 cm. of water shows almost immediately the deposition of some alumina, and the same thing happens when the alumina is first precipitated by ammonia and then dissolved by just enough sulphuric acid to effect the solution. The difficulty of so adjusting the acidity of the solution that no alumina shall be deposited and no titanium held up, is obvious. Test paper is of no aid in the case, since the reaction of alum itself and aluminic sulphate is acid. Under the circumstances it is hardly surprising that Knop should revert from the troublesome, and only under remote con

* Jahrb. f. Min. 1876, p. 756; Zeit. f. Kryst. u. Min., i. 58.

ditions more exact, method of separation by boiling to the convenient but indubitably inaccurate process of parting by the use of sodic hydrate. Knop employs both the dry and the wet method, the fusion of the oxides in sodic hydrate, and the treatment of the salts in solution with sodic hydrate in excess; but both had long before . been generally discarded, and the single experiment of acidulating the alkaline filtrate from a precipitate obtained by the treatment of a pure titanium salt according to either mode, and then making alkaline with ammonia, is enough to prove their worthlessness.

In an endeavor to secure a better means of separating titanium from aluminum I have followed two lines of experimentation ; — the one based upon the observation that, under properly controlled conditions, titanium is completely precipitated and alumina held in solution when an alkaline orthophosphate, strongly acidified with formic acid, is added to the solution of salts of these elements; the other, upon the solubility of alumina and the insolubility of titanic hydrate in sufficiently strong boiling solutions of acetic acid. Two preparations of titanium were used as test material in the course of the work; -the solution of the fusion in sodic bisulphate of the hydrate precipitated by ammonia from the fluoride of titanium and potassium itself made from rutile and carefully purified by recrystallization; and the solution obtained by treating the carefully prepared double fluoride with sulphuric acid, evaporating to dryness, redissolving in sulphuric acid and diluting with water. The second mode of preparation is the better, because the amount of alkaline sulphate present in the test solution is much smaller. At first, the strength of the solution was fixed by precipitating weighed amounts by ammonia, carefully washing, iguiting and weighing the precipitate; but in the course of the work it became plain that the precipitation by means of ammonic acetate, or by ammonia with the subsequent addition of acetic acid in distinct excess, yielded more precise results. On this account, therefore, the determinations of the standard of the solutions employed in the later work were made by the acetic acid process, and the determinations by ammonia were corrected to correspond. In some of the experiments bearing upon this point, the results of which are given in the figures below, precipitations by ammonia were made in the cold and the liquid then heated to boiling; in those in which acetic acid was used just in excess, this reagent succeeded the ammonia at once, and the boiling followed; when precipitation was effected in presence of large amounts of acetic or formic acid, the acid was added in the amount intended, sodic acetate in quantity more than that necessary to decom

pose the sulphates present was introduced, and the clear solution heated to boiling, and kept boiling for a minute or two. The acetic acid employed was the G. P. article of commerce, and contained thirty-five per cent of absolute acid. The amounts of it used - 20%, 30%, 40%, 50% by volume correspond to 7%, 10.5%, 14%, 17.5% of absolute acid. The formic acid contained twenty-seven per cent of pure acid, and when it is said that there was in solution 5.4% of it, the absolute acid is meant; but it was the weaker acid, to the amount of 20% of the entire volume, which was actually used. The experiments were made in sets, and so appear in the record, in consequence of changes (due to slight depositions from time to time) in the strength of the test solution, which necessitated redeterminations of standard.

It will be noticed in the examination of these figures that parallel determinations usually agree very closely. The amounts of titanic oxide indicated by those experiments in which the precipitation was made by ammonia, are much in excess of those in which acetic acid was added subsequently. Thus the difference between (1), (2) and (3), (4) amounts to more than three per cent of the total amount of the former; that between (5), (6) and (7), (8), to a little less than two per cent; that between (9), (10) and (11), (12), to about two and a half per cent; and a correction of more than two per cent must be applied to (13), (14) to bring them to correspondence with (15), (16), (17), (18). The difference between (21), (22), (23) and (24), (25), (26), (27) is about one per cent, and the smallness of this figure in comparison with the differences previously noted is apparently explicable by the fact that the solution of titanium employed in the last determinations was prepared by the second of the methods mentioned above, and carries a smaller amount of alkaline sulphate. The tendency of titanic hydrate to include the sulphates of the alkalies is not strange in view of the well-known conduct of aluminic hydrate under similar circumstances, but the amount thus held is rather surprising. The experiments in which different proportions of free acid were introduced go to show, very strikingly, that, if acetic acid exerts any solvent action whatever upon the precipitate thrown down by boiling the acetate, that action is very slight. Thus, between the mean of (24), (25) and that of (26), (27), the one set precipitated by ammonia and treated before boiling with just a distinct excess of acetic acid, the other pair thrown out of a large volume, 700 cm.3, one half of which was acid of 35% strength, by boiling, we find a difference of but 0.0007 grm., and between the mean of (15), (16) and that of (17), (18), the difference (magnified five times by reference to 50 grm. portions) is 0.0010 grm. In (25), too, we have an experiment in which the weighed precipitate was fused in sodic carbonate, dissolved, and again precipitated as before and weighed, the two weights differing by 0.0003 grm. Moreover, the filtrates from the precipitates thrown out in presence of an excess of acetic acid, when neutralized with ammonia, failed invariably to show the smallest precipitate, and in direct experiments upon the sensitiveness of the reaction it was found that on the addition of 0.0005 grm. of titanic oxide in solution to 100 cm.3 of 35% acetic acid carrying a little sodic acetate a distinct precipitate appeared almost immediately on boiling. It is plain, therefore, that so far as concerns the purpose in hand the insolubility of the titanium precipitate in acetic acid may be taken as absolute. The