Further, to carry out the purification, the blue powder (oxide of iridium) is re-dissolved in aqua regia, evaporated to dryness, re-dissolved in water, and filtered.

The dark-coloured solution thus obtained is slowly poured into a concentrated solution of soda and mixed with hypochlorite of soda, and should remain as a clear solution without any perceptible precipitate, and subjected in a distilling apparatus to a stream of chlorine gas, should not show a trace of ruthenium when hydrochloric acid and alcohol are introduced into the receiver. In this operation the chlorine precipitates the greater part of the iridium in a state of blue oxide, which after being collected, washed, and dried, is placed in a porcelain or glass tube, and subjected to the combined action of oxide of carbon and carbonic acid obtained by means of a mixture of oxalic with sulphuric acid gently heated.

The oxide of iridium is reduced by the action of the gas leaving the oxide of iron intact, the mass is then heated to redness with bi-sulphate of potash (which will take up the iron and any remaining trace of rhodium) and after subjecting it to many washings with distilled water, the residue is washed with chlorine water to remove any trace of gold, and finally with hydrofluoric acid, in order to take out any silica which might have been accidentally introduced with the alkalies employed or have come off the vessels used.

The iridium after calcination at a strong heat in a charcoal crucible, is melted into an ingot, and after being broken up and boiled in hydrochloric acid, to remove any possible trace of iron adhering to it through the abrasion in breaking up, should possess if perfectly pure a density of 22:39; but, as iridium prepared even with the utmost care will still contain minute though almost inappreciable traces of oxygen, ruthenium, rhodium, and possibly iron, the highest density I have yet attained is 22.38.

Alloy of Iridio Platinum.

This compound metal possesses physical properties of great value, forming a beautiful example of the effect of a careful combination of the opposite characteristics of its component parts. Thus, the extreme softness and expansiveness of pure platinum and the brittleness and excessive hardness of pure iridium, produce, by combination in judicious proportions, a perfect and homogeneous alloy, possessing the necessary mean of these properties to render it suitable for many important purposes, amongst others that of the special object to be attained to meet the requirements for an unalterable standard metal, for which it is peculiarly adapted.

In the manufacture of the prototype metres and the geodesique rules (each 4 metres in length) ordered from my firm by the Comité Internationale des Poids et Mesures, the Association Géodésique In

ternationale, and the French Minister of War, I proceeded in the following manner with the platinum and iridium prepared as described above.

Operating upon a charge of 450 ounces of platinum and 55 ounces of iridium, I commenced by melting these metals together and casting into an ingot of suitable shape, which I then cut into small pieces with hydraulic machinery. After re-melting and retaining in a molten condition under a powerful blast of oxygen and common gas for a considerable time, I re-cast and forged at an intense white heat under a steam hammer, the highly polished surfaces of which were cleaned and polished after each series of blows-when sufficiently reduced it was passed through bright polished steel rollers, cut into narrow strips, and again slowly melted in a properly shaped mould, in which it was allowed to cool. I thus obtained a mass of suitable shape for forging, perfectly solid, homogeneous, free from fissures or air-holes, and with a bright and clean surface at bottom and sides as at top. At the first forging a bar was obtained 35 centims. long, 7.5 wide, 2.5 thick, which weighed

In air

In water at 70° F.....

15.105 grms.
14:405

Showing a density at zero of 21.522

A third of the bar was cut off and the larger portion again forged to a length of 95 centims., width 2.5, thickness 2.0, which weighed—

In air

In water at 60° F.

10.814 grms.
10.315

Showing a density at zero of 21-648

This was then passed through highly polished rolls until of a length of 4,010 centims. 21 millims. in width, and 5 millims. thick, to which a perfectly rectangular form was subsequently given by drawing it through a series of plates, and thus prepared the rule was in a condition to receive the beautiful polish of which this alloy is susceptible.

After passing it through each hole the metal was annealed by means of a jet of gas and oxygen to a heat just below melting point, and each time throughout after forging, rolling, and drawing was exposed to the action of melted borax, and boiled in concentrated hydrochloric acid to remove any possible trace of adherent iron or other impurity.

A piece cut from the end and presented to the French Academy of Science gave the following results:

thus proving that the necessary processes of annealing at a high temperature had caused it sensibly to resume its original density.

The analysis gave

Density at zero, calculated after No. 1 analysis....21 510
Density at zero, calculated after No. 2

....21 515

thus coinciding perfectly with the practical results obtained.

Messrs. Leon Brunner Brothers, of Paris, who had submitted this material of the géodésique rule to a great number of mechanical experiments, communicated the result of their observations to M. H. Sainte-Claire Deville, thus ::

"MONSIEUR,

"Paris, 27 Août, 1878.

"La division de la règle géodésique, que nous faisons pour l'Association Géodésique Internationale, est terminée depuis quelques jours.

"Nous avons pensé que vous ne seriez pas mécontent d'apprendre que cette operation a parfaitement réussi, et que c'est au métal que nous attribuons la facilité avec laquelle nous avons pu l'exécuter.

"Le platine iridié de M. Matthey est incontestablement supérieur au platine ordinaire, pour la confection des règles divisées. Il est exempt de ces pailles qu'on rencontre toujours dans ce dernier, et se laisse polir au charbon. On peut, sans danger, enlever les rébarbes des traits et les conserver très beaux. Le platine ordinaire ne peut-être poli qu'au papier à émeré, et l'on est toujours exposé à gâter la division quand on procéde à l'ébarbage. C'est là un inconvénient très-grave.

"Nous ne pouvons que vous remercier, Monsieur, d'avoir mis à notre disposition un metal qui modifie singulièrement les difficultés

qu'on rencontre dans la fabrication d'une règle géodésique, et nous vous prions de recevoir l'assurance de nos sentiments les plus distingues.

"BRUNNER FRERES."

In the year 1876 the suggestion was made to supersede the rectangular form by a tubular one, and I was requested to produce one of the following dimensions: Length, 1,002 centims.; exterior diameter, 37 millims.; interior diameter, 35 millims. ; with rounded ends, one having an extension of small tube 4 millims. exterior diameter, 2 millms. interior diameter, 40 millms. long, which I did by the system of tube making with autogenous joints adopted by me with excellent results for the last 20 years, employing for the purpose an alloy prepared as above described. These proved to be so satisfactory that I have since made others, both round and square, of various dimensions, as lately shown at the Paris Exhibition.

Iridio-platinum alloy has now been proved to possess the following among many advantages for standard rules and weights :

It is almost indestructible, has extreme rigidity, especially in the tube form, and a most beautifully polished surface can be obtained upon it; its coefficient of elasticity is very great, whilst for standard weights its high density is a valuable quality, and for these I should indeed recommend an alloy of not less than 20 per cent. of iridium. I lately made at the request of M. H. Sainte-Claire Deville a cylinder 40 millims. by 40 millims. of such an alloy, which showed by analysis the following proportions):

With such a high density its coefficient of elasticity is 22 200000, one of the highest known, whilst its malleability and ductility are almost without limit.

The volume of the kilogram thus prepared is only 46.266 cub. cemtims., it displaces 2 267 cub. centims. less than the kilogram of the archives of France, and on this account, as on many others, is of course preferable.

The results I have arrived at in preparing alloys of higher grades, viz., 25—30—40 and 50 per cent. of iridium, are as follows:

The alloy of 20 per cent. iridium is, as I have stated already, malleable and ductile.

25 per cent. can only with great difficulty and waste be worked

into sheet and wire when heated at low temperature. 30 per cent. and 40 per cent. with great difficulty only at a temperature little less than melting point, being brittle when cold, but with a grain of great beauty and fineness.

50 per cent. I have as yet failed to work up into forms other than castings beyond what I can effect by pressure when in a semi-fused condition.

The general results of my work on this alloy would lead me, therefore, to make the following recommendations.

For the manufacture of standard rules to use an alloy of not less than 85 per cent. platinum and 15 per cent. iridium, adopting the tubular form.

For the standard weights to use an alloy of not less than 80 per per cent. platinum and 20 per cent. iridium, adopting the form now generally made.

Finally, following the expression of the great French chemist, M. Dumas, I hope by these labours "d'avoir enriché l'outillage scientifique d'un alliage doué des propriétés précieuse."

III. "On the Reversal of the Lines of Metallic Vapours." No. VI. By G. D. LIVEING, M.A., Professor of Chemistry, and J. DEWAR, M.A., F.R.S., Jacksonian Professor, University of Cambridge. Received March 27, 1879.

The experiments described in the following communication were made with the electric arc, and in lime crucibles,* or in crucibles of a highly calcareous sandstone, kindly supplied to us by Messrs. Johnson, Matthey, and Co., as described in our fourth communication on this subject; but for some of them we used, instead of a galvanic battery, a magneto-electric machine producing a much more powerful current and a much longer arc. The experiments with this machine were made, through the kindness of Dr. Tyndall, at the Royal Institution, and we are indebted to Messrs. Siemens both for the working of the machine and for sparing to us the services of a skilled engineer, in

* In our first paper on this subject, communicated in February, 1878, when referring to the experiments of Lockyer and Roberts ("Proc. Roy. Soc.," xxiii), we mentioned that they employed the combined action of a charcoal furnace and an oxyhydrogen blowpipe, but omitted to mention that they used a lime chamber after the model of Stas. Referring to fig. 1 in our communication of February 12, 1879, where the use of an oxyhydrogen blowpipe in a lime block is represented, we disclaim any novelty in the use of lime; the difference between our experiments and theirs consisting in this, that we use the continuous spectrum from the hot walls of our crucible, instead of an external independent source of light, as a background against which the absorbent action of the vapours is seen, in the same way as we had previously used iron tubes, and now use the electric arc.