242 On the Chemical Constitution of Saline Solutions. [Mar. 19, 



there are few cases in which no change is noticed. 2ndly, generally the 

 amount of light transmitted is diminished to a small extent by some of 

 the more refrangible, the less refrangible, or both lands of rays being 

 obstructed. 3rdly, there is frequently a complete difference in the 

 nature of the transmitted light. Anhydrous salts not decomposed, 

 hydrated compounds not dehydrated at 100 C., and salts which do not 

 change colour on dehydration, give little or no alteration in their spectra 

 when heated. 



Solutions of hydrated salts, and most notably those of haloid com- 

 pounds, do change ; and the alteration is, if not identical with, similar to 

 that produced by dehydration and the action of dehydrating liquids, such 

 as alcohol, acids, and glycerine, on the salts in crystals or solution. 



A particular instance of the action of heat on an aqueous solution is 

 that of cobalt chloride, which gives a different series of dark bands in 

 the red part of the spectrum at different temperatures, ranging between 

 23 C. and 73 C. Band after band of shadow intercepts the red rays as 

 the temperature rises, till finally nothing but the blue are transmitted. 

 Drawings of six different spectra of this remarkable nature have been 

 made. The changes are most marked between 33 and 53, when the 

 temperature may be told almost to a degree by noting the appearance of 

 the spectrum. Though to the unaided eye cobalt bromide appears to 

 undergo the same change, yet, as seen with the spectroscope, it is not of 

 so curious a character, the bands being not so numerous. 



With cobalt iodide a band of red light is transmitted at low temperatures ; 

 the band of light moves towards the opposite end of the spectrum, with rise 

 of temperature, until it is transferred to such a position that it consists of 

 green rays only. In this instance the change to the eye is more striking 

 when seen without the spectroscope, because the mixtures of red, yellow, 

 and green rays, which are formed during the transition, give rise to very 

 beautiful shades of brown and olive-green. Thus a saturated solution at 

 16 C. was of a brown colour, at 10 C. it became of a fiery red and 

 crystals separated, at +10 reddish brown, at 20 the same, at 35 

 Vandyke brown, at 45 a cold brown tint with a tinge of yellowish green, 

 at 55 a decidedly yellowish green in thin layers and yellow-brown in 

 thick, at 65 greenish brown, thin layers green, and at 75 olive-green. 

 An examination of this cobalt salt has shown that there are two distinct 

 crystalline hydrates the one, formed at high temperatures, has the 

 formula Co C1 2 ,2H 2 O, and is of a dark green colour ; the other, which 

 contains a much larger proportion of crystalline water, Co C1 2 , 6H 2 O, 

 is produced at a low temperature, and its colour is generally brown, 

 in cold weather inclining to red. 



The action of heat on solutions of didymium is characterized by a 

 broadening of the black lines seen in the spectrum, more especially of 

 the important band in the yellow ; and in the case of potassio-didymium 

 nitrate, this is accompanied by the formation of a new line. In the case 



