ActionofLow Temperatures on Supersaturated Saline Solutions. 295 



the full aperture of 9 inches when an eyepiece-spectroscope with 

 direct vision is employed. (Secchi says, "the aperture of the re- 

 fractor becomes thus reduced by more than half its surface, yet 

 notwithstanding the light is so intense that it greatly exceeds 

 that which one obtains with interposed direct-vision prisms near 

 the eyepiece. The dispersion is so considerable that it is at 

 least six times that which I have obtained with the most pow- 

 erful spectroscopic eyepiece, and even that of the spectroscope 

 without slit, with cylindrical lens, which I originally employed.") 



The price of this object-glass spectral apparatus amounted to 

 525 Bavarian florins, equal to £4i6. 



With a certain loss of light there could, however, be given to 

 this object-glass spectral apparatus for analyzing stellar light a 

 disposition for observing by direct vision. Within these few days 

 an object-glass spectral prism with direct vision has been com- 

 pleted, composed of crown- and flint-glass, which has completely 

 borne out what was anticipated from it. This arrangement has 

 an aperture of 34 French lines ( = 3*02 English inches) ; and its 

 dispersive power is equivalent to that of a flint-glass prism the 

 refracting angle of which is 26°. 



XXXVI. On the Action of Low Temperatures on Supersaturated 

 Saline Solutions. By Charles Tomlinson, F.R.S* ; 



TWO years ago I had the honour of bringing before the 

 Chemical Section of the British Association an account of 

 an experiment in which it was shown that crystals of magnesic 

 sulphate, sodic sulphate, and one or two other salts do not neces- 

 sarily act as nuclei to their supersaturated solutions. My object 

 was to show, by an extreme case, that if a body which is usually 

 a most powerful nucleus be made chemically clean, the solution 

 adheres to it as a whole, and there is no separation of salt. 



1 propose to-day to bring before the Section another case, which 

 seems to me as extreme as the former one. It is intended to 

 illustrate this position, namely that, in the absence of a nucleus, 

 highly supersaturated solutions reduced to the zero of Fahren* 

 heit's scale, or from that to —10°, solidify into unstable hy- 

 drates rather than crystallize; and on exposing such solidified 

 solutions to a temperature of about 32°, they rapidly liquefy into 

 clear, bright, supersaturated solutions without any separation of 

 salt. This effect may be produced any number of times, pro- 

 vided the solution be preserved from the action of nuclei or 

 carriers of nuclei, such as the air. The only precaution to this 



* Communicated by the Author, having been read before the Chemical 

 Section of the British Association at Liverpool, September 20, 1870. 



