A Comprehensive Treatise on Inorganic and Theoretical Chemistry

226 INORGANIC AND THEORETICAL. CHEMISTRY
2£ times the hardening effect of iridium on platinum, and it also increases the
electrical resistance 2£ times as quickly. An alloy with over 10 per cent, of osmium
is difficult to -work owing to its hardness and lack of ductility—vide Table IV.
The annealing must be done under reducing conditions or the osmium burns off.
F. Zimmermann discussed the hardness of the alloys. H. Remy and B. Schaffer
observed that osmium-platinum alloys are not very active catalysts in the reaction
between hydrogen and oxygen, although separately the metals are active. H. Remy
and H. Gonnington studied the subject. G. R. Levi observed that the presence
of 10 per cent, of osmium almost doubled the catalytic activity of platinum on
hydrogen dioxide. E. Haagn used an alloy with 40 to 60 per cent, of ruthenium,
35 to 5O per cent, of osmium, and 5 to 15 per cent, of platinum for tipping the nibs
of fountain pens. F. E. Carter said that the platinum-palladium-osmium alloys,
formerly used for jewellery, were abandoned in favour of the platinum-palladiumrhodium
alloys because of the volatilization of osmium when heated—a subject
discussed by C. M. Hoke.
G. Rose, 25 and A. Breithaupt described a native platinum-iridium alloy from
the Urals. V. Rekschinsky discussed the separation of osmiridium (q.v.) from
metals of the platinum group. J. J. Berzelius observed that equal weights of
platinum and iridium form a brittle alloy which can be welded, and that the alloy
with a small proportion of iridium is ductile and harder than iridium, and more
resistant to high temperatures, and to chemical reagents. H. Morin, and A. Gaudin
prepared alloys with 10 per cent, of iridium and found them to be malleable, and
not to tarnish when employed as metallic mirrors on copper. B. S. Jacobi discussed
the use of an alloy with 20 per cent, of iridium for medals. He said that the alloy
can be cold-worked, and that it is hard, and is only slightly affected by aqua regia.
O. J. Broch and co-workers, and H. St. C. Deville and H. Debray described the
preparation of the platinum-iridium standard measure for the Comite International
des Poids et Mesures. F. E. Carter noted iridium can be worked at high temp.,
but a little platinum induces brittleness. Iridium is employed to harden platinum
to enable it to be used in chemical ware, electric work, and jewellery. In medium
hard jewellery, the platinum contains 5 per cent, of iridium, and in hard jewellery,
10 per cent. The limit of workability is 30 to 35 per cent, of iridium. The alloys
are solid soln., and any coring in the crystal grains can be rectified by annealing.
Segregation does not usually occur. F. Korn, and E. A. Smith discussed the
application of the alloys in the jewellery trade, etc. H. St. C. Deville and H. Debray
found the sp. gr. of alloys with 10, 15, 33*3, and 95 per cent, of iridium to be respectively
21-615, 21-618, 21-874, and 22-384. T. Barratt and R. M. Winter gave for
the thermal conductivity of alloys :
17° ibo°
Per cent, iridium . . . io 15 20 10 15 2O
CaIs. per cm. per sec. per degree . 0074 0-056 0042 0075 0-059 0042
O. Feussner and L. Miiller, and L. Muller measured the f.p. of alloys of the two
metals, and the results are summarized in Fig. 73. F. E. Carter said that the
alloys do not oxidize above 1150°, but iridium slowly volatilizes from them ; a
film of oxide forms when the alloy is cooled in the range 1150° to 900°. K. Friederich
studied the magnetic properties of the alloys. They darken superficially when
heated within the range 900° to 1100°, presumably owing to the formation of an
oxide ; at a higher temp., the oxide is decomposed and the surface regains its
colour. Iridium itself acquires a bluish film under similar conditions. The alloys
begin to lose weight above 900° owing to the volatilization of the iridium oxide.
Hence for high temp, gravimetric work iridium is objectionable in platinum
crucibles, and in thermocouple work. E. Haber studied the resistance of platinumiridium
electrodes in the electrolysis of hydrochloric acid; and J. Liike and R. Fricke,
the decomposition of nitrous oxide by glowing wires of the alloy. B. S. Srikantan
studied the reaction H2-I-CO2^CO-I-H2C) on platinum-iridium alloys ; H. Remy'