110 



Mr. C. Tomlinson on Supersaturated 



[Jan. 25, 



flask was opened; crystallization of the ordinary salt set in from the 

 surface, and the temperature rose from 44° to 65°. 



Thus one more hydrate is added to those already known as belonging 

 to this remarkable salt. It doubtless contains less water than the seven- 

 atom hydrate ; but I know of no method of testing its hydration, since its 

 existence depends upon the low temperature, and shelter from the action 

 of nuclei. In this way it resembles the various hydrates described in my 

 paper in the ' Transactions/ 



The solution was next made twice as strong as before, that is, two 

 parts of Glauber's salt were dissolved in one part of water, and after boiling 

 and filtering and reboiling as before, the flask was set aside to cool. 

 When the thermometer marked 42°, the flask was put into the freezing- 

 mixture. At 38° a few transparent octahedra were thrown down, and the 

 heat-currents thereby liberated delayed the cooling. In fourteen minutes 

 it reached 26°, and the transparent crystals at the bottom became opaque 

 white. The thermometer was stationary during some minutes at 26°, 

 when it began again to descend ; but on agitating the flask in the freezing- 

 mixture, crystals of the opaque-white salt were formed, and the tempera- 

 ture regained 26°, the solution above being bright and clear, and still 

 supersaturated. In a few minutes crystallization set in from the surface, 

 and the thermometer rose from 26° to 53°, the whole being now solid. 



These opaque crystals resemble in texture newly formed white lead ; 

 and at whatever temperature they may be formed below 26°, their forma- 

 tion causes the thermometer to rise to 26°, and that, too, in solutions of 

 1 part, 2 parts, or 3 parts salt to one of water. This opaque salt is some- 

 times amorphous, and then it covers the surface of the flask like thick 

 whitewash. This effect occurs when the flask is much agitated in the 

 freezing-mixture. 



The same flask (2 salt to 1 water) was reboiled without any addition of 

 water, so that the solution was really stronger than that indicated. At 

 40° there was a fall of transparent anhydrous crystals. The solution was 

 now purposely cooled very slowly, so that in half an hour it descended 

 only 3°, namely, to 37°. There was now a considerable increase of the 

 anhydrous salt so as to cover the bottom of the flask, and to rise a little 

 way up the sides. The flask was transferred to a freezing-mixture at 10°; 

 when at 33° the anhydrous salt became opaque, doubtless from the fixa- 

 tion of a portion of water less than that required for the formation of the 

 seven-atom salt. At 24° opaque tufts and fern-like crystals were formed. 

 At 22° there was a sudden and copious deposit of this opaque-white hy- 

 drate ; the thermometer rose to 26°, and then suddenly to 52°, when the 

 whole mass was solid . 



It is commonly supposed that the rise in temperature consequent on 

 the solidification of a supersaturated solution is dependent on its mass ; 

 that when this is considerable, the rise in temperature is so too, but that 

 when the mass is small there is but little heating. This does not accord 



