Armand de Quatrefages, Georg Hermann Quincke, Theodor Schwann, and Jean Servais Stas were balloted for and elected Foreign Members of the Society.

The following Papers were read:

I. "On the Thermal Conductivity of Water." By J. T. BOTTOMLEY, Lecturer in Natural Philosophy and Demonstrator in Experimental Physics in the University of Glasgow. Communicated by Professor Sir WILLIAM THOMSON, LL.D., F.R.S. Received March 11, 1879.

(Abstract.)

The experiments described in this paper were undertaken at the instance of Sir William Thomson and by a method devised by him.

The liquid whose thermal conductivity is to be determined is heated from above, to avoid convection currents. Two methods of heating have been used. In one, a horizontal steam chamber is applied at the top of the water or other liquid; and, steam being continuously passed through the heating chamber, the surface of the liquid under experiment is kept at a very high temperature, and heat is conducted from above downwards. In the other method a large quantity of very hot water is deposited on the top of a mass of cold water, mixing being prevented by a simple contrivance; and the heat of this superincumbent layer is conducted downwards through the colder water below.

The experiments have been carried on in very large vessels, or tanks, in order to avoid disturbance by means of loss of heat at the sides. It is intended, at the suggestion of Professor Clerk Maxwell, to observe the loss of heat by the sides under given circumstances, and to estimate, from results of such experiments, the probable error due to this loss.

In the experiments three principal thermometers are employed; together with a fourth, whose object is merely to show when heat begins to be lost at the bottom of the layer of fluid experimented on. When this loss commences the experiment is at an end. The other three thermometers are used thus:-First there is a thermometer with a bulb 30 centims. long. It is placed vertically; and its object is to show the average temperature from top to bottom of the layer of fluid bounded by horizontal planes passing through the top and bottom of its bulb. The rise of this thermometer in any time shows the quantity of heat that has passed into the stratum occupied by it in that time. The other two thermometers are placed with their bulbs horizontal, and one at a known distance vertically above the

other. They indicate the temperatures of the layers in which they are placed.

Now, if we know the difference of temperatures of two sides of a stratum of a liquid during any time, and the quantity of heat conducted across the stratum during that interval of time, we can calculate the thermal conductivity of the liquid by means of a wellknown formula.

The result arrived at by the experiments described, is that the thermal conductivity of water may be taken at from 0022 to 00245 in square centimetres per second.

Some experiments have been made on the thermal conductivity of solution of sulphate of zinc, a solution which happened to be convenient for preliminary trials. The specific heat of solution of sulphate of zinc at different densities, which it is necessary to know for comparison as to thermal conductivity of that liquid with water, has been determined.

Experiments are now being carried on on this subject with the assistance of a grant from the Government Fund of 4,000l.

II. "The Preparation in a State of Purity of the Group of Metals known as the Platinum Series, and Notes upon the Manufacture of Iridio-Platinum." By GEORGE MATTHEY. Communicated by F. A. ABEL, C.B., F.R.S. Received March 19, 1879.

In this paper it is not my intention, nor should I be able, to refer generally to the results of work in the various branches of platinum metallurgy carried out by my firm, who, as is well known, have been associated with the development of this special field of industry from its earliest infancy; but I shall confine myself simply to that section of it upon which my personal attention has of late years been specifically concentrated in order to meet and comply with the requisition of the Bureau Internationale des Poids et Mesures, the Section Françaises de la Commission Internationale du Mètre, and of l'Association Géodésique Internationale (all of them important scientific committees, formed with the object of arriving at an accurate and definite solution of the long agitated question of standard weights and measures), and also at the demand of the French Minister of War, for an alloy the best adapted for the manufacture of the international metre and kilogram standard, and the geodesique rule; and in my endea vour to solve this difficult problem I have had the great advantage of being able to consult those distinguished men, M.M. Henri Sainte Claire Deville and Henri Debray, of Paris, and have also had the

benefit of the excellent and valued advice of M. Stas, the celebrated Belgian chemist, to all of whom the scientific world owe so much, and to whom I desire to offer my warmest thanks.

In a paper of this kind it would be superfluous for me to enter into any of the already published details concerning the existence and collection of what is known as platinum-dust or mineral. It is sufficient for me to observe that the six metals (of which platinum is the chief) usually found more or less in association in their native state, present characteristics of interest beyond their metallurgical utility, which are, perhaps, worth alluding to en passant. It is, for instance, a curious fact that the group should consist of three light and three heavy metals, each division being of approximately the same specific gravity-the heavier being (in round figures) just double the density of the lighter series.

Thus we find osmium, iridium, platinum forming the first division, of the respective specific gravities of 22:43, 22:39, 21:46; whilst ruthenium, rhodium, and palladium are represented by the figures 11:40, 11:36, 11, the average densities of the heavy and light divisions thus being respectively 22:43 and 11.25.

But a more interesting and important classification is what I may designate as a first and second class series, from the more important view of their relative properties of stability. Thus platinum, palladium, and rhodium form the first or higher class, not being volatilizable in a state of oxide; iridium, osmium, and ruthenium forming the second or lower class, their oxides being more or less readily volatilized.

The oxide of iridium is affected at 700 to 800° C., and entirely decomposed at 1,000°, whilst osmic and hyporuthenic acids are volatilized at the low degree of 100°, the latter exploding at 108°. The chlorides of these metals can be sublimed at different temperatures (as also the protochloride of platinum).

I now propose to give a short description of the methods I have employed for preparing the pure platinum and iridium necessary for the manufacture of the alloy, which I call "iridio-platinum," and it is upon the distinguishing characteristics above-mentioned that my method of separation is chiefly founded.

Platinum.

The preparation of this metal to a state of purity is an operation of extreme delicacy. I commence by taking ordinary commercial platinum; I melt this with six times its weight of lead of ascertained purity, and, after granulation, dissolve slowly in nitric acid diluted in the proportion of 1 volume to 8 of distilled water. The more readily to ensure dissolution, it is well to place the granulated alloy in porcelain

baskets such as are used in the manufacture of chlorine gas for holding the oxide of manganese. When the first charge of acid is sufficiently saturated, a fresh quantity should be added until no more action is apparent; at this stage the greater part of the lead will have been dissolved out together with a portion of any copper, iron, palladium, or rhodium that may have been present. These metals are subsequently extracted from the mother-liquors, the nitrate of lead by crystallization, and the remaining metals by well-known methods.

The metallic residue now obtained will be found in the state of an amorphous black powder (a form most suitable for further treatment), consisting of platinum, lead, and small proportions of the other metals originally present the iridium existing as a brilliant crystalline substance insoluble in nitric acid. After digesting this compound in weak aqua regia, an immediate dissolution takes place of the platinum and lead, leaving the iridium still impure, but effecting a complete separation of the platinum.

To the chloride of platinum and lead after evaporation is added sufficient sulphuric acid to effect the precipitation of the whole of the lead as a sulphate, and the chloride of platinum after dissolution in distilled water is treated with an excess of chloride of ammonium and sodium, the excess being necessary in order that the precipitated yellow double salt may remain in a saturated solution of the precipitant. The whole is then heated to about 80°, and allowed to stand for some days; the ammonio-chloride of platinum will settle down as a firm deposit at the bottom of the vessel, whilst if any rhodium, as is generally the case, is present, the surface liquor will be coloured a rose tint, occasioned by a combination of the salts of the two metals.

The precipitate must be repeatedly washed with a saturated solution of chloride of ammonium and subsequently with distilled water charged with pure hydrochloric acid. This is necessary for its purification. The small quantity of the double salt which will be taken up and held in solution is of course recovered afterwards. Rhodium may still exist in the washed precipitate, which must therefore not be reduced to the metallic state until its separation is completed, and this is best effected by mixing with the dried compound, salts of chloro-platinate and chloro-rhodiate of ammonia, bi-sulphate of potash with a small proportion of bi-sulphate of ammonia, and subjecting to a gradual heat brought by degrees up to a dull red in a platinum capsule, over which is placed an inverted glass funnel. The platinum is thus slowly reduced to a black spongy porous condition freed from water, nitrogen, sulphate of ammonia, and hydrochloric acid, the rhodium remaining in a soluble state as bi-sulphate of rhodium and potash, which can be dissolved out completely by digesting in boiling distilled water; a small quantity of platinum will have been taken up in a state of sulphate, but is regained by heating the residue (obtained

on evaporation) to redness, at which heat it is reduced to the metallic condition, the rhodium salt remaining undecomposed.

By the method above described the platinum is freed not only from rhodium, but from all other metals with which it may have been contaminated, and is brought to a state of absolute purity, of the density 21:46, the highest degree obtainable.

Iridium.

In the preparation of this metal when intended to be used for the manufacture of iridio-platinum alloy, I have arrived at freeing it to the utmost possible extent from all its associate metals, except platinum, disregarding the presence of the latter; the proportion of which, once determined, would only form matter of calculation in the final operation of mixing my alloy.

In practice, the purest iridium which can be obtained from its ordinary solution (deprived of osmium by long boiling in aqua regia and precipitated by chloride of ammonium) will almost invariably contain traces of platinum, rhodium, ruthenium, and iron.

I fuse such iridium in a fine state of division with ten times its weight of lead, keeping it in a molten state for some hours, dissolve out the lead with nitric acid, subject the residue to a prolonged digestion in aqua regia, and obtain a crystalline mass composed of iridium, rhodium, ruthenium, and iron, in a condition suitable for my further treatment. By fusion at a high temperature with an admixture of bi-sulphate of potash, the rhodium is almost entirely removed, any remaining trace being taken up together with the iron in a later operation. The iridium so far prepared is melted with 'ten times its weight of dry caustic potash, and three times its weight of nitre, in a gold pan or crucible; the process being prolonged for a considerable time to effect the complete transformation of the material into iridiate and ruthenate of potash, and the oxidation of the iron; when cold, the mixture is treated with cold distilled water. The iridiate of potash of a blue tinge will remain as a deposit almost insoluble in water, more especially if slightly alkaline, and also the oxide of iron.

This precipitate must be well washed with water charged with a little potash and hypochlorite of soda until the washings are no longer coloured, and then several times with distilled water.

The blue powder is then mixed with water strongly charged with hypochlorite of soda, and allowed to remain for a time cold, then warmed in a distilling vessel, and finally brought up to boiling point until the distillate no longer colours red, weak alcohol acidulated with hydrochloric acid.

The residue is again heated with nitre and potash water charged with hypochlorite of soda and chlorine, until the last trace of ruthenium has disappeared.