SIR W. CROOKES ON THE PHOTOGRAPHIC SPECTRA OF METEORITES. 
425 
photographic effect of known percentages of chromium under the conditions obtaining 
in the aerolite spectra. 
Two electrodes were made as follows :— 
Yt 2 0 :i .70 
Kaolin. 10 
Nickel. 15 
Chromium. 5 
100 
And to this was added an equal weight of pure silver powder. The arc spectrum 
was then photographed under conditions similar to those of the aerolites. 
It was seen that the chromium groups were still the most prominent lines in the 
spectrum, those of nickel being comparatively faint. 
Keeping the other ingredients constant and gradually reducing the proportion of 
chromium several more experiments were made, and it was found that when the 
chromium had been so reduced that it was only 0‘16 per cent., the lines A 4862'02 
and A 4870'9 were no longer visible ; a further experiment with electrodes containing 
only O'l per cent, chromium distinctly showed the other two groups. 
These experiments show that it is easy to detect the presence of chromium 
in an arc between electrodes that contain only 5 parts Cr in 10,000. 
It is not easy to see why in subjecting the two elements nickel and chromium 
to the heat of the electric arc it should produce so much more intense atomic 
disturbance in one case than in the other—the melting-points of the two elements 
are not very different—it may be, however, that the volatilisation points differ. 
I have pursued this matter in connection with the examination of a large number of 
meteoric irons (siderites) that will form the subject of a further communication.* 
Although these experiments only make it possible to form an approximate 
estimate of the amount of chromium present in these aerolites it would seem to lie 
between 0'6 per cent, and O'l per cent. 
In the previous experiments the proportion of nickel which was kept the same in 
all mixtures was evidently in considerable excess of that contained in any of the 
* Without anticipating this communication on siderites, I desire to point out the remarkable fact that 
although the chromium lines appear, sometimes very strongly, in all my aerolite spectra they do not 
appear in any of the siderites I have so far examined—except in one instance, that of “ Zacatecas.’’ 
Although of course the nickel lines are always visible—from the spectroscopic examination of a number 
of specimens of iron, to which I had added decreasing amounts of chromium, I found that the dominant 
lines of chromium were quite visible even when the amount present was no more than 0 - 0175 per cent- 
This is due to the fact that chromium is absent from the nickel-irons but almost universally present in 
the aerolites in the form of chromite. In Aubres, &c., there is probably more chromite than nickel-iron. 
In the magma which produced meteorites all the Cr was converted into oxide or sulphide, giving rise to 
the mineral chromite or daubreelite and leaving no Cr for the nickel-iron. 
