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that should be most carefully observed. And I may mention here that while at low temperature, a good surface illumination of the ribbon is necessary, at the higher temperatures observations are best made by the light emitted by the platinum itself. Illustrative of the use of the meldometer I may quote here approximate determinations of the melting points of the felspars, which have afforded the following numbers:—
Adularia, .. .. .. 1176
Oligoclase, .. .. 1220
Sanidine, .. .. .. 1140
Mierocline, .. .. .. 1175
Labradorite, .. .. .. 1230
These bodies melt rapidly at some 20 degrees higher. The melting points of adularia, mierocline, and albite cannot be distinguished from one another. Adularia may be distinguished from albite, however, by the development of bubbles at higher temperatures, as already referred to. It is, of course, possible that there will be small variations of melting point from one sample to another. These bodies well illustrate the phenomena of viscosity. It is instructive to compare the behaviour of these viscous silicates with the behaviour of such a mineral as halite, which runs out suddenly at 772° C. In spite of the uncertainty introduced into the melting points by the phenomenon of viscosity the discriminative value is evident even in the case of this related group of minerals. Thus by comparison we can pick out the substances adularia, mierocline, and albite from the others; or oligoclase, and labradorite, and in a third division sanidine. The phenomena accompanying melting will differentiate the individuals of the two first divisions from one another. When we pass to substances less closely related there is, of course, found a great diversity of melting points, although indeed it is remarkable what a preponderating number of mineral bodies possess melting points ranging within the comparatively narrow limits of 900° C. to 1500° C. (which last is above the melting point of quartz). But this matter is to be considered in the second part of this Paper.
Although the majority of mineral bodies have melting points ranging over 900° C, some, such as the sulphides, arsenides, and some of the elements can even be brought into the state of vapour upon the meldometer. When this is the case we find that this apparatus affords a means of obtaining sublimates much transcending the blow-pipe (need either with carbon or aluminium) in delicacy, range, and purity. Sublimates may, in fact, easily be obtained from substances which treated in the blowpipe would appear to afford none, as will appear. The mode of procedure is as follows :—A circular cover-glass—not too thin—grasped in a forceps, is held horizontally above the platinum ribbon, as the temperature is being raised. If the sublimate is one which oxidizes, we obtain the oxidized sublimate alone; or the unoxidized sublimate bordered by the oxide, according as we hold the glass further or nearer to the ribbon.
The more volatile elements often afford as sublimates both the element and an oxide of it. The elemental sublimate may often be obtained nearly pure by suitably regulating the temperature. Thus, at low temperatures arsenic sublimes as a rich gray-black sublimate, showing the mirror when viewed through the glass. At a higher temperature, especially if the glass is held at a distance of three or four centimetres above the ribbon, the white oxide—the trioxide— only is obtained. Sometimes both element and oxide are together on the one glass, affording an "eye," the pupil of which is the element. This results from the screening action of the outer parts of the ascending column of vapour, the central parts being, in fact, sublimed in the sbsence of oxygen, which is all absorbed in the outer layers of the vapour. Magnesium affords similar eyes surrounded by the white oxide, or in many cases veiled over by it, so that the dark metal is only seen through the glass. The great avidity of this element for oxygen is shown in this fact. Tin also affords "eyes." Thallium throws a rich black velvety sublimate, fringed with deep ash-red (the oxide?). This is a very beautiful sublimate, but very fleeting, the black soon fading into a pale grayish-brown colour. If immediately enclosed from the air it retains its original tints a longer time. Cadmium may also be sublimed as the metal and its oxide. Indium affords a white sublimate, dashed with pink and yellow.
While the metal is thus sometimes obtained with the oxide it more generally affords the oxide only. This is the case, so far as I have observed, with vanadium, lead, wolfram, bismuth, tellurium, zinc, and antimny. But, again, sometimes the element appears to sublime without oxidizing. Thus silver affords a gray-black veil of the metal, iridescent where thinly deposited. Gold is also sublimed. Sulphur is another esse, the oxide being a gas at atmospheric temperatures. Mercury gives a sublimate of a grey colour, consisting of globules of the element.
As regards compounds, the command we have over the temperature in the meldometer enables many very distinct separations to be effected. Thus, dealing with realgar, at low temperatures, the substance is sublimed unaltered in a rich yellow sublimate. Somewhat higher a decomposition is effected, the free arsenic showing as a white sublimate of the oxide round a yellow eye of sublimed realgar. As the temperature rises the effect is more and more that proper to arsenic only, the liberated sulphur not appearing; but the eyes remain most generally distinctly touched with realgar. Orpiment behaves in a similar manner. Pyrargyrite, a compound of silver and antimony sulphides, throws off the antimony first in a rich white sublimate of the oxide, touched more or less with a pale pink cloud, probably the unaltered compound. A bead of silver is left upon the ribbon, which ultimately volatilizes to the gray-black sublimate of silver. Clawthalite, the selenide of lead, affords first a sublimate of selenium, a fine ash-red; this then becomes veiled over and intermixed with the rich yellow and whites of the lead oxides, so that a very beautiful marbling is produced, which shows stronger tints of red seen from the back of the glass than from the front.
Many such effects are seen in a similar order with the blow-pipe, but are not produced with the ease, certainty, and cleanliness obtaining with the meldometer. Tests may very conveniently be applied to these sublimates as they repose upon the glass, in the knowledge that the only addition to the original substance can be oxygen. Sublimates also may be obtained from very minute quantities of the substance. This i6 an advantage in more ways than one. Many of the bodies mentioned above—as arsenic, tin, zinc, bismuth—attack the platinum ribbon at high temperatures, and cut it if more than a very small quantity be used. The metals iron, gold, and silver again amalgamate with it. If it is desired, however, to obtain considerable quantities of the sublimates of the more volatile elements, it is possible by laying upon the ribbon a small slip of thin mica, and upon this the substance, to volatilize considerable quantities of it. Lest it be urged that this apparatus is an expensive one to use owing to the necessity of using platinum, I may mention that a spool of the pure platinum ribbon, having a length of 1700 centimetres wound upon it, is supplied to me by Messrs. Johnson and Matthey, of Hatton Garden, London, at a cost of twenty shillings. This lasts an indefinite time, as a couple of centimetres serves most generally for many observations, and in the case of silicates, may then often be cleaned, so as to be again serviceable, by treatment in acids. That the platinum, however, volatilizes slowly at high temperature is shown by the fact that if fragments of quartz be heated upon it nearly to their melting point, they will be found to become covered with minute crystals of platinum upon their upper surface.1
But the meldometer is capable of affording sublimates which the blowpipe very certainly will not reveal. Thus, for example, tourmaline affords a pale whitish-yellow sublimate, the nature of which I have not determined; and enstatite volatilizes at the highest temperatures obtainable, very nearly, giving a pale brown sublimate. An addition may be made to this form of the meldometer, which will permit of sublimates being obtained in the absence of free oxygen. This is an annular chamber of light brass open at both sides, and cut into at two points, so that it can be set down upon the base plate of the meldometer, the two slots admitting the forceps without making contact with them. It is provided with two small projections which, entering perforations in the base plate, retain it in position. It is shown dotted in fig. 1. Across the upper surface of this box the cover-glass to receive the sublimate is placed, and the sublimate thrown upon it in the usual manner. A tubular at one side permits of connexion with a supply of COa or other gas which, flowing in very slowly under slight pressure, and escaping around the forceps, replaces the air and stops its entry. If the sublimate is a very heavy one —that is one which falls downward when generated—a second, smaller cover-glass is supported above the floor of the chamber upon a little tripod twisted out of wire, so as to be just beneath the forceps. In this way sublimates of realgar and arsenic may be made to afford the unoxidized substances. As it is necessary in order to obtain an abundant sublimate that there should only be a slow motion of the atmosphere around the platinum, guards are arranged, attached to the base plate beneath the forceps, which close the slots provided in the ring when this is in its place. It is further well to loosely close the opening between the limbs of the forceps with a little cotton wool. A very flow current of inert gas then effectually secures "that no air enters while the sublimate is being taken. However, in the use of this arrangement, the sublimate is generally obtained deposited in patterns —often very regular—upon the glass, duo to slow swirling currents within the chamber.
1 See Nature, xuv. (1891), p. 124.
Before passing from the subject of the secondary uses to which this form of the meldometer may be put, it remains to add that much of the pyro-chemical work done with the blowpipe may with greater ease and delicacy be effected upon the meldometer. Thus, glasses with microcosmic salt or with borax may be made readily upon the ribbon, the colours produced being well seen, and that, too, however deep in tint, where they thin out at the ends along the bright platinum strip.
Again, abandoning the use of the ribbon, we may substitute a platinum wire carrying a loop at its centre, and clamping it in the forceps, form beads of great beauty of the usual form from the action of the hot wire. These may be observed, under the microscope, while hot. Changes of colour, often so characteristic, are very distinctly observed through the microscope directed upon the platinum ribbon. For example, the changes of tint of a glass formed of copper oxide (CuO) with borax, coating the ribbon, as the temperature is slowly raised, is from a fine blue through every gradation of tint to a greenish yellow. The command we possess over the temperature enables these successive changes to be very readily observed. Similarly, the oxidizing effects of the blowpipe may be obtained by addition of oxidizing substances, such as potassium nitrate. Thus, as with the blowpipe, a glass formed of the sesquioxide of cerium and microcosmic salt which is a pale yellow when hot, passing to colourless when cold, may by the addition of KNOj be intensified to a vivid yellow when hot, to colourless when cold. By the use of reducing agents deoxidation may of course, be effected. In this way a mixture of cupric oxide with carbonate of soda and cyanide of potassium yields, first the lower cuprous oxide as a transparent red crystalline body, and finally the metal which alloys with the platinum. The most minute quantities may be used. I have not had leisure to develop this application of the meldometer, but would call the attention of those versed in pyrochemistry to the facilities it offers for minute and clean work. The form now described possesses the added advantage that the temperature at which any phenomenon is occurring can be determined with as much facility as we read a thermometer.
Measurement Of Temperature On The Meldometer.
The temperature of a homogeneous conductor of constant section, heated by the passage of a current, may be considered uniform through