SIR W. CROOKES ON THE PHOTOGRAPHIC SPECTRA OF METEORITES. 
423 
Launton (from Bicester, Oxfordshire). 
The whole of the 5 grm. of material available quickly burnt out and only 
short exposures were possible. The photographed spectrum extended from 
A 3302 to A 5896, and includes in addition to the lines of iron 2 aluminium, 
2 calcium, 21 chromium, 2 magnesium, 3 manganese, 20 nickel, 2 potassium, 
6 sodium. 
Mount Brown (from Evelyn County, New South Wales). 
The photographed spectrum of this aerolite extends from A 2599 to A 6643. 
and includes in addition to the lines of iron 3 aluminium, 3 calcium, 
28 chromium, 11 magnesium, 3 manganese, 55 nickel, 6 sodium. 
Mount Dyrring (from Durham County, New South Wales). (See Plate 20.) 
The lines found in the photographed spectrum of this aerolite extends from 
A 2482 to A 4652, and includes in addition to the lines of iron 25 chromium, 
17 magnesium, 3 manganese, nickel, 2 potassium, 6 silicon, 2 sodium. 
Nammianthal (from South Arcot, Madras, India). 
The photographed spectrum of this aerolite extends from A 2716 to A 4652, 
and includes in addition to the lines of iron 2 calcium, 23 chromium, 
3 magnesium, 3 manganese, 50 nickel, 2 sodium. 
Parnallee (from Madras, India). 
The photographed spectrum extends from A 2788 to A 5896, and includes 
in addition to the lines of iron 1 aluminium, 24 chromium, 3 magnesium, 
3 manganese, 50 nickel, 2 potassium, 6 sodium. 
Before discussing in detail the composition of these aerolites as revealed by their 
arc spectra it is well to record one or two points of general importance. Keeping 
the current and voltage constant the number of lines recorded in the arc spectrum of 
a compound substance depends in large measure upon the time-length of the 
“ exposure ” and the sensitiveness of the photographic plate. For instance, if in an 
exposure of say five minutes there are a mass of lines, having some that are very 
much over-exposed and some that are only just visible, by making another exposure 
of the same material for double the time the very faint lines would appear stronger, 
and others that in the shorter exposure were not visible woidd come into view, and 
so on. But in practice a limit is fixed, governed partly by the demands of the 
strong lines and partly by the amount of the material at one’s disposal. This latter 
limitation, the amount of material available, is very potent when dealing with 
such substances as rare meteoric stones. 
From an examination of the whole set of aerolite spectra it appears that the 
proportion of nickel to iron is generally constant. There is a nickel line at 
A 3619'391 and an iron at A3618'919; these two lines form a close pair, and in 
twenty-seven out of the thirty spectra they remain in the same relative intensity, 
