This accounts for the fact that these colours are more strongly scattered by the granulations of the films.

A collodion surface only reflects about 5 per cent. of the incident energy; and it was found impossible to account for the strong colours seen in the reflected light, by compounding the feeble stream of light from the collodion with the powerful stream coming from the metal.

It appeared, however, that the observed effects could be accounted for, if the somewhat arbitrary assumption were made that the granulated surface reflected more strongly than a smooth surface. As I have already said, the granulations are too small to interfere with the regular reflexion of light, the scattering being selective, so to speak, i. e. confined to the waves which, owing to interference, are compelled to traverse the film a number of times.

One

The assumption above referred to appeared to be too arbitrary to make without some experimental evidence, and experiments were therefore made to determine the effect of the "frilling" of the film on its reflecting power. of the faces of a 60° prism of crown glass was flowed with collodion of the same dilution as that used in the preparation of the coloured films. It showed in reflected light interference-colours, which, however, were very much diluted with white light, owing to the small difference between the refractive indices of the two media. In working with the film on silver it was found that, if the colours did not appear at once, as soon as the film dried, they could be brought out by breathing on the film, the deposit of moisture being advantageous to the formation of the granulations. It was always possible to intensify the colours in this way. The film deposited on the surface of the prism was treated in this way, one half of it being screened from the deposit of moisture by a plate of glass. As soon as the moisture had evaporated, it was found that the reflecting power of the surface had been greatly increased, the film appearing almost as bright as a half-silvered surface.

The increase in the brilliancy of the reflected light was about three-fold, as was shown by covering the unfrilled portion with a sheet of thin glass, which about equalized the intensities. In other words, the frilled collodion-surface regularly reflects white light, of an intensity very nearly equal to that of light reflected from thin glass surfaces.

On examining the granular surface with polarized light, it was found that the angle of maximum polarization was in the neighbourhood of 63°, which would make its refractive index about 1.96. The polarizing angle of the smooth collodion

was about 56°, the corresponding refractive index being 1.48.

An attempt was made to determine whether the granulation gave rise to elliptical polarization, the abnormal value of the refractive index suggesting the properties of the surface-films, which play such an important part in the theory of elliptical polarization. No decisive results were obtained, for though the phenomenon was found, it seemed impossible to eliminate the component reflected from the collodionglass surface, which, as I have shown, may, by interference with the component reflected from the air-collodion surface, give rise to an elliptical vibration.

The interferometer failed to show any change in the refractive index as the result of frilling, which indicates that the effect is confined to the surface. A film deposited on glass of such thickness as to produce a shift of half a fringe width (sodium light) was frilled by moisture, one half being protected by a glass plate. No shift was found at the line of demarcation, as would have been the case if the refractive index of the film had been raised from 1-48 to 1.96 throughout its entire thickness.

It is my plan to make a further study of the apparent effect of the granulation on the refractive index of the surface, by the method of total reflexion.

XLVI. Disintegration of the Platinum Metals in Different Gases*. By L. HOLBORN and L. W. AUSTIN†.

HE disintegration of the platinum metals at high tempera

tures appears to be primarily due to their high meltingpoints. The phenomenon which has been observed in many connexions in the case of platinum also occurs with rhodium, palladium, and in a still greater degree with iridium. Observations on the disintegration of these metals in air have been carried out by one of the authors ‡. The metals were used in the form of strips 3 mm. wide which were heated electrically. The "black temperature," S, was measured by means of the optical pyrometer §, the temperature Celsius, t,

*This article in somewhat different form was presented to the Berlin Academy. Sitz. Ber. der Berl. Akad. p. 245 (1903).

† Communicated by the Authors.

↑ L. Holborn and F. Henning, Sitz. Ber. p. 938 (1902). SL. Holborn and F. Kurlbaum, Sitz. Ber. p. 712 (1901).

being calculated from the empirical formula

t=1.157 S-67.2.

It was found that the disintegration increased very rapidly with increasing temperature, iridium showing a loss of weight of 11.8 mg. in an hour at 1210° C., 72 mg. at 1670°, and 277 mg, at 2130°. At 1670° platinum and rhodium showed about the same loss in weight, while iridium lost approximately ten times as much. The pure metals showed no decrease in disintegration during the longest heating (3 hours), the loss appearing to be proportional to the time. Changes were observed, however, in the cases of the platinum-iridium alloys, undoubtedly due to the changes in the composition at the surface on account of the more rapid disintegration of the iridium. Silver and gold heated to 100° below their melting-points showed no certain loss in weight. We have now extended this work to cover the disintegration in different gases.

Observations on the disintegration of glowing metals in different gases thus far have been confined to platinum and palladium. Elster and Geitel *, and Nahrwold † found, contrary to Berliner ‡, that platinum showed very little disintegration in hydrogen. Stewart § investigated platinum and palladium, and observed that both metals showed little or no disintegration in nitrogen; that in hydrogen, platinum showed no loss in weight and palladium much less than in air. He made no observations with oxygen, but cites an observation by Kaufmann to the effect that in oxygen the disintegration of platinum is six times as great as in air. According to Stewart, the disintegration of platinum in air decreases with decreasing pressure, while in the case of palladium it increases. Emich has studied the action of hot platinum in air and in nitric oxide, and has shown that oxygen is taken up by the platinum from the air and from the decomposed nitric oxide.

In the present work the same methods were followed as in the former, except that now it was necessary to decrease the width of the metal strips in order to bring them to the required temperature with a smaller current. These narrower strips, especially in the cases of the very hard rhodium and iridium, were very difficult to produce of uniform width and

* J. Elster and H. Geitel, Wied. Ann. xxxi. p. 126 (1887).

+ R. Nahrwold, ib. xxxv. p. 116 (1888).

A. Berliner, ib. xxxiii. p. 291 (1888).

§ W. Stewart, ib. lxvi. p. 88 (1898); Phil. Mag. [5] xlviii. p. 481 (1899).

||F. Emich, Sitz.-Ber. der Wiener Akademie, ci. [2 b], p. 88 (1892).

without irregularities. The different strips could not therefore be properly compared with one another, but this was not of great moment as the chief object

of the work was the comparison of the disintegration of the same strip under different conditions.

The 75 mm. long strips were clamped to 6 mm. thick copper wires and the whole introduced into a glass tube (fig. 1) 12 cm. in diameter, by means of a ground-glass connexion. The copper wires were cemented into the glass with sealing-wax. All the other external connexions of the tube were rendered tight with mercury. tube was also provided with a side tube containing P20, to be used at low gas pressures. The current was introduced through mercury cups in which the ends of the copper wires rested.

The

The metal strips were too narrow for direct comparison with the optical

Fig. 1.

pyrometer, therefore a 3 mm. wide, electrically heated platinum band was brought to the required temperature. Against this the metal strip in the tube was projected, and heated until it attained the same degree of brightness.

Contrary to the observations on the broad metal strips, a diminution of the disintegration with the time was often observed. This was due to the fact that the narrow strips, on account of the disintegration, soon became thinner in the central hottest zone; and if this was kept at a constant temperature by reducing the current, the length over which the disintegration took place became constantly shorter. In the cases where the disintegration was most rapid, we have therefore not used the highest possible temperatures and have also reduced the usual 30 minutes time of heating, in order to prolong the uniformity of the strips and to enable a greater number of comparisons to be made. Another reason for reducing the amount of the disintegration was that the transparency of the tube was rapidly destroyed by the metallic deposits which had to be frequently removed between the observations.

The weighings were made on a balance having a sensibility of about three divisions for 0.1 mg., and are certain to 0.01 mg.

The observations on each metal were made in the following order :

1. At atmospheric pressure, the tube being left open at top and bottom to allow a free circulation of air.

2. With the tube pumped out to a water vacuum.

3. With a slow stream of commercial oxygen or nitrogen passing slowly through the tube.

4. With the tube filled with hydrogen at different pressures.