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MR. W. CROOKES OK RADIANT MATTER SPECTROSCOPY": 
very small quantity of water—not sufficient to dissolve the one-hundredth part of it. 
The mass was thrown on a filter, and the small quantity of clear liquid which came 
through was precipitated with ammonic oxalate. The resulting white precipitate was 
ignited with sulphuric acid, and tested in the radiant matter tube. For the sake of 
comparison a portion of the calcic sulphate remaining on the filter was also put in a 
radiant matter tube. The sulphate from the aqueous extract gave the citron band far 
more brilliantly than the calcic sulphate from the filter. I found, however, that it 
was impossible, by any amount of washing or boiling out, to deprive the calcic sulphate 
of all power of giving the citron band, although it was possible in this way to weaken 
its intensity considerably. 
Experiments with calcic sulphate. 
15. Supposing that the substance giving the citron band formed a sulphate more 
soluble in water than calcic sulphate, it was anticipated that repeated washings with 
cold water would extract some of it, which might then be detected more easily. 
About four pounds’ weight of commercial plaster of Paris, which showed very faint 
traces of the citron band, were mixed with water and rapidly poured on a large filter. 
Before the mass solidified a slight saucer-like depression was made in the upper part, 
and a few ounces of water were poured on. This ran through slowly, and it was then 
poured back and the exhaustion repeated several times. The aqueous extract was 
then evaporated to dryness, ignited with sulphuric acid, ground in a mortar with 
small successive quantities of water, the liquid boiled, filtered, and precipitated, first 
with ammonia, and the filtrate with ammonic oxalate. These precipitates both showed 
the citron band very fairly, far more intensely than it was seen in the original calcic 
sulphate. The green and red bands were also visible. 
The same mass of plaster of Paris was then washed, as before, with a little dilute 
hydrochloric acid passed through several times, and this extract was treated in the 
same way by evaporation and extraction with water, and the filtrate precipitated, first 
with ammonia, and then with ammonic oxalate. In these precipitates the citron band, 
together with the green and red bands, were much more brightly manifest than in the 
precipitates from the aqueous extract. 
Wide distribution of the citron band-forming body. 
16. These experiments are conclusive in proving that the citron band is not due to 
calcium, but to some other element, probably one of the earthy metals, occurring in 
very minute quantities, but widely distributed along with calcium, and I at once 
commenced experiments to find a more abundant supply of the body sought for. 
Amongst other substances tested I may note the following as giving a more or less 
decided citron band in the spectrum when treated with sulphuric acid in the manner 
indicated above (10) :—Crystallised barytic chlorate, heavy spar, common limestone, 
strontic nitrate, native strontic carbonate, crystallised uranic nitrate, commercial 
