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THE

bearing compounds appears to be influenced by the structure of the molecule, so that these bodies fall into groups defined by very different sublimation temperatures. (a) Compounds containing the Molecules RAs, RASS, R2As3, or R3A84(?).—Eleven mineral species and seventeen specimens of minerals reputed to carry their arsenic in some one of these molecules have been examined. The temperature of earliest sublimation varied about 440°; one only dropping to 360°, and one rising to 5150, and then deflagrating violently. This group includes such minerals as arsenopyrite, FeAsS; glaucodot, (Co,Fe)ASS; leucopyrite, FeAs, Fe2As3; and a rammelsbergite or chloanthite to which an analysis on the apophorometer assigns the composition Ni3A84. (This requires confirmation).

I was at the time rather surprised at the apparent sameness of temperature at which certain very different mineral species decomposed, but contented myself by pointing out the probable importance of temperature measurements, and by indicating a method of effecting such measurements. This method-that of determining the temperature in terms of the current which heats the hob upon which the substance reposes-has proved very satisfactory; the current and temperature maintaining a rectilinear relation to at least 1200° C.

Since the writing of my first paper I have been able to observe the temperatures of sublimation of a certain number of typical minerals, and, although more observations are desirable, the approximate agreement among the results obtained, as well as the delay which must attend the acquisition of further material, induce me to give a brief account of what I have observed. I shall refer to three volatile constituents only in this present note: antimony, arsenic, and tellurium. It will be understood that the temperatures given below are approximate only, their value chiefly residing in their comparability. Every precaution was taken to ensure this by making the conditions alike in every case, and allowing a fixed time for the sublimate to appear.

1. A large number of antimony bearing minerals carry their antimony in the stibnite or antimonite molecule, Sb2O3. The mineral stibnite sublimes as Sb406 or Sb204 at 480°. Of thirteen other mineral species examined containing this molecule, the temperature of sublimation fell between 430° and 520°. Two others containing this molecule rose to 550° and 600° respectively. Both these gave up their antimony with violent deflagration, a phenomenon suggesting superheating above the normal temperature of volatilisation and oxidation. Three other minerals carrying the antimony as R3Sb, or RSb, sublime within the limits 490-520°. The native element or the chemically purified element sublimed at 510° or a little higher.

From these results it will be inferred, contrary to what, I think, would have been expected, that a compound of the form nRS,mSb2S3 becomes unstable in presence of oxygen at a temperature which seems unaffected by the nature of R or the values of n or m. Thus zinkenite, PbS, Sb2S3) sublimes at 475°, and tetrahedrite, 4(Cu2S), Sb2S3, at the same temperature. A specimen of polybasite, 9Ag2S, Sb2S3, sublimes at 495°, &c. It would further appear that other antimony-bearing molecules besides the sesquisulphide also break up at this temperature. It is also remarkable that the temperature of sublimation may be lower than that of the free element, although I think we must conclude that it is the temperature of sublimation of this last which is the controlling factor throughout.

2. The difference between the behaviour of arsenic and

(c) Compounds containing ASS or Loosely Attached Arsenic. These are all in very close agreement, yielding arsenious oxide at about 140°, the extremes being 135-150° as observed over seven species and eight specimens. The native or chemically prepared element sublimed at 145°; realgar, AsS, at 145°; and some substances of the smaltite-chloanthite-rammelsbergite section showed a sublimate at 140°. This may be due to the presence of loosely attached arsenic or possibly to the presence of the skutterudite molecule often assigned to certain of these bodies.

Among the arsenic bearing minerals it will be seen that there is traceable a fairly regular advance in stability as the volatile atom becomes more subordinate in the molecule. This general fact seems well supported; but whether this subordination is conditioned entirely by the purely numerical ratio of the atoms in the molecule or whether the atomic weights have an influence there are not enough observations to discriminate with certainty. Again, differences in chemical affinity may well play a part. It is premature to enter into these questions. It will, however, be readily understood what use can be made of the above relations in assigning structural formulæ to complex volatile substances. Thus, arsenopyrite is written either as FeS2, FeAs2, or as FeAsS. The first of these is negatived and the second supported by the stability of the mineral over the interval 190-250°, and its decomposition within the range of group (a).

It is necessary to observe that a substance may, by this means of classification, be assigned to a group lower than its general formula entitles it to, if a certain amount (possibly quite small) of a less stable molecule is present. Ön the other hand, if it sublimes at a temperature above that proper to the reputed formula there is ground for calling the latter in question. Some analyses which I made on this basis resulted in revising the chemical formula of the substance and correcting erroneous labelling. It is sometimes found that the sublimation cannot be completed at, or anywhere near, the group temperature, but that the rise of temperature must be carried into that of the succeeding group. A copious sublimation may then suddenly occur. This phenomenon may indicate the original presence of the molecule of the higher group, or it may result from the formation of the latter after a certain amount of the volatile element has been removed. A succession of melting-points, crystallisations, and colour changes is often noticed attending the gradual decomposition of a mineral on the apophorometer or meldometer. Either of the above explanations may apply here also. But in some cases it is easy to show that molecules of

differing stabilities must have existed constitutionally; for, if not, at each higher stage a lesser quantity of sublimate is to be expected-a geometrical decrease; whereas I have found that the successive quantities of sublimate removed may be increasing. And here I may add that a certain amount of arsenic subliming at relatively high temperatures (1000° or over), after a long interval of cessation, suggests the existence of atomic combinations whose stability places them in higher groups. It would appear justifiable to use this fractional sublimation of a substance as a means of assisting the determination of a constitutional formula.

3. The tellurides examined have, for lack of material, been few in number. Tellurium, as I showed in my first paper (loc. cit.), comes off as a low-temperature black sublimate of TeO, or as a high-temperature white sublimate of TeO2. Now I have found that two tellurides of the formula RTe gave no black sublimate, but only the white at a temperature of from 800° to 900°; and five tellurides having the formula RTe2 gave the black sublimate at 460-545°, and the white sublimate at 750-900°. Tellurium itself gave a black sublimate at about 500°, and a white sublimate at 850. According to these few observations the tellurides appear capable of grouping in somewhat the same manner as the arsenides.

Whether further experiment will enable the antimony. bearing substances to be separated into temperature-groups remains to be seen; but on the available results the difference of behaviour of the three volatile elements is remarkable, especially when considered in conjunction with the physical properties of the elements themselves. Other questions of interest suggest themselves: Will the artificial alloys reveal temperature grouping according to their atomic proportions? If my conclusions are sustained, the quantitative estimation of sublimates and observation of their temperatures of evolution will, I believe, be found useful in approaching questions of atomic stability in certain cases and in discriminating among constitutional formulæ arising from analysis.

arranged the width of the annular space between the ends of the two tubes is less than the diameter of the inlet tube when the mercury flows up this rather than round it. It is found quite possible to arrange this so that the mercury could invariably be raised half an inch in the tube, outside which no liquid appears. The space is, however, capable of passing sufficient gas when the temperature and therefore the mercury falls. This bevelling conforming to the meniscus of the mercury, assists materially in maintaining that form and preserving its continuity.

The quantity of mercury utilised is adjusted by the usual side tube. A fine screw working inside a large one (but

AN IMPROVEMENT IN GAS THERMOREGULATORS.

By A. WHITAKER.

A SERIOUS difficulty which has been encountered in designing gas thermo-regulators is the peculiar properties of a glass mercury contact. When such is introduced as the "cut off" in the instrument the mercury tends to seal externally rather than enter the comparatively fine orifice of the inlet tube, a greater quantity of mercury being therefore required for a given rise in level. Further, unless the tube-end is quite horizontal, the gas escapes from the elevated parts often in the form of bubbles, causing the flame to flicker. Again, except in cases where no attempt is made to use either a small orifice here or the capillary tube of the usual type, the mercury has a tendency to separate or adhere to the glass; the portion removed from action, having to be replaced, causes an elevation in the mean temperature of the bath. The interposition of rubber meets these difficulties to some extent, but introduces other disadvantages.

The following is offered as a solution :

Unless the joint at A is very well made (that is, regular and symmetrical), the end of the capillary should be countersunk (by grinding) as flat as is compatible with its object. The regulator here described exhibited an angle of 135°. The flow of mercury is by this means considerably facilitated, and is approximately equal from all points.

The end of the inlet tube is ground internally to a cone, the edges being either sharp or slightly expanded, and brought very near to the prepared capillary. If properly

FIG. 1.

not too large) will be found more convenient than a reservoir and tap, giving considerable range with fine adjustment. This is only necessary if a large bulb be used.

The apparatus is quick in action and consequently easy of adjustment. If the bath be heated to the required temperature and the screw then turned till the flame is cut down, the mean temperature of the bath will be within a twentieth of a degree of the desired one, Half an hour is sufficient for exact adjustment, other instruments requiring occasional attention for two or three days if regulation to