392 Royal Society : — 



compounds, do change ; and the alteration^, if not identical with, 

 similar to that produced by dehydration and the action of dehydra- 

 ting 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 solu- 

 tion is that of cobalt chloride, which gives a different series of 

 dark bands in the red part of the spectrum at different tempera- 

 tures, ranging between 23° C. and 73° C. Band after band of sha- 

 dow 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 spec- 

 trum. 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 tempe- 

 ratures ; 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 spectro- 

 scope, 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 crys- 

 tals separated, at -f 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 Cl 2 , 2H 2 0, and is of a dark green 

 colour; the other, which contains a much larger proportion of 

 crystalline water, Co Cl 2 , 6H 2 0, 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 of didymium acetate, which decomposes 

 with separation of a basic salt, the lines thickened on heating. 



Thermo-chemical Experiments. 



Eegnauld (Institut, 1864 ; Jahresbericht, 1864, p. 99) has shown 

 that on diluting a saturated solution of a salt, as a rule there is an 

 absorption of heat ; but in one or two cases he noticed that heat 

 was evolved. The change in colour that takes place on the dilution 

 of saturated solutions of cobalt iodide, cupric chloride, bromide, and 

 acetate is very remarkable. There is every likelihood that this phe- 

 nomenon is due in each case to the formation of a liquid hydrate. 



