894 
MB. W. CROOKES ON RADIANT MATTER SPECTROSCOPY: 
a warm place : a slight flocculent precipitate formed. This was filtered, and the 
filtrate re-concentrated. The clear strong solution should now contain nothing but 
barium, strontium, and calcium, with traces of elements from previous groups which 
might be soluble in the precipitants employed or in the aramoniacal salts present (for 
we know that the word insoluble applied to a precipitate is not an absolute term, and 
in minute analysis allowance must be made not only for the slight solubility of 
precipitates in the reagents present, but also for the power possessed by most 
precipitates of carrying down with them traces of soluble metallic salts from solution). 
Besides these, it was possible that a hitherto unrecognised element might be present, 
to which the citron band was due. By the ordinary process of analysis I could, 
however, only detect the presence of calcium and strontium. 
8. The concentrated ammoniacal solution was added to an excess of a boiling- 
solution of amnionic sulphate, and the whole was set aside for twenty-four hours ; the 
precipitate which had formed was filtered off and washed with a saturated solution of 
ammonic sulphate. The precipitate was found to consist of strontic sulphate. On 
testing this in a radiant matter tube the citron band was very decided, although much 
fainter than in the original oxalate. The filtrate was diluted largely, heated, and 
precipitated with a hot solution of ammonic oxalate; it was then allowed to stand for 
some time, when a bulky white calcic oxalate came down. This was filtered and 
washed. Tested in the radiant matter tube, after ignition and treatment with 
sulphuric acid, it gave the citron band, far exceeding in brightness the spectrum of 
the original oxalate. 
9. So far all the chemical evidence went to show that the band-forming substance 
was calcium, and further tests tried with the purified oxalate confirmed this inference. 
Every analytical test to which it was subjected showed lime, and nothing but lime ; 
all the salts which were prepared from it resembled those of lime, both physically and 
chemically; the flame spectrum gave the calcium lines with extraordinary purity and 
brilliancy; and finally, the atomic weight, taken with great care, came out almost the 
same as that for calcium, 39'9 as against Ca 40. 
10. I now sought for the citron band amongst other calcium minerals. The 
preliminary testing was simple. The finely powdered mineral was moistened with 
strong sulphuric acid, the action being assisted by heat, and the mass was then raised 
to dull redness (4). It was then put into a radiant matter tube and the induction 
spark passed through after the exhaustion had been pushed to the required degree. 
Treated in this manner most native compounds of lime gave the citron band. A 
perfectly clear and colourless crystal of Iceland spar converted into sulphate gave it 
strongly, native calcic phosphate less strongly, and a crystal of arragonite much more 
brightly. A stalactite of calcic carbonate from the Gibraltar caves gave the band 
almost as well as calcite, as also did cinnamon stone (lime alumina garnet), iron slag 
from a blast-furnace, commercial plaster of Paris, and most specimens of ordinary 
burnt lime. 
