Chemical Constitution of Saline Solutions. 291 



At the temperature of 100° a basic salt separated; and while this decomposition 

 occurred, the black lines thickened, and those bands which form a group in the 

 green became confused. Only four measurements could be made at this tempera- 

 ture on account of the turbidity of the liquid. 



Iodide and bromide of didymium were prepared, but, as they readily decom- 

 posed, they were unsatisfactor}" salts to work with. 



Didumium Sulphate. — A finely crystallized specimen, sparingly soluble in water. 

 A saturated solution was made at 20°. It was photographed with the four-prism 

 spectrograph, the plate being so adjusted, that the zero of the scale used in measur- 

 ing the spectra was at the edge of the plate, and the red end of the spectra carefully 

 focussed. 



The illumination was sunlight, and the solution was contained in a test-tube 

 half an inch in diameter. This salt is not very soluble, as it required more than 

 forty times its weight of water to dissolve it at 20°. 



ABSOEPTION BAND, I. 



X A. 



At 20°, . . . 5822 to 5750 



At 100°, . . . 5834 to 5750 



ABSOlirTION BAND, II. (not so strong as I.). 



Bays transmitted and tlie spectrum terminates — 



X 

 At 20°, . . . 3344 

 At 100°, . . . 3372 



The spectrum begins at \ 6198. The second band is much weaker than the 

 first, and is less distinct at 20° than when the solution is hot. 



It will be seen from these measurements and by inspection of more recent 

 photographs, that at 100° the absorption bands have widened slightly, this 

 being particularly noticeable in the first one, and slightly in the second, where it 

 is seen to be shifted, apparently, but this is due to the band having widened on 

 the weaker or less refrangible side when the solution was hot. The most remark- 

 able change in the spectrum caused by rise of temperature is the increased 

 absorption of the ultra-violet rays, which in the cold solution were entirely 

 transmitted as far as the sun's spectrum could be photographed. 



Having, in June last, the opportunity of photographing the absorption spectra 

 of neodym-ammonium nitrate and praseodym-ammonium nitrate from small 



