8 Mr. F. Guthrie on Salt Solutions and Attached Water. 



and Mitscherlich, whose analyses, however, are not in good 

 accord, that of Lowitz being as much as 10 per cent, different 

 from the calculated percentage of the bi hydrate. Ehrenberg 

 and Frankenheim state that the same hydrate is produced when 

 a dilute solution of chloride of sodium evaporates at 15° C, and 

 that it suddenly converts itself into the cubical anhydrous salt 

 and water. At —22° C. the whole of this hydrate appears to be 

 removed. The gradual impoverishment of the mother-brine 

 was tested by maintaining a brine, at first saturated, for half an 

 hour at the successive temperatures —10°, —16°, and —21° to 

 — 22° C. The brine had been previously kept at 0° for an hour. 

 The original brine contained 26*2724 of NaCl per cent. It was 

 the mother-brine from each crop of crystals which was subjected 

 to the lower temperature. 



per cent. 

 Saturated brine at 0° contained .... 26*2724 of NaCl. 

 Mother-liquor after keeping at — 10° „ 24*6528 „ 



-10° „ 24-6187 „ 

 -16° „ 24-1182 „ 

 -21° to 22° 23-8874 



It is clear, therefore, that down to —21° the solidification im- 

 poverishes the brine, — a result which might be due to the for- 

 mation of the bihydrate or the precipitation of the anhydrous 

 salt, but which is inconsistent with the formation of ice alone. 

 The determinations were made by weighing 10 or 12 grammes 

 of the brines, when at the atmospheric temperature, into long- 

 necked flasks, evaporating and heating to about 300° C. 



§ 12. A quantity of brine which had been thus impoverished 

 by being kept for an hour at —21° to —22° was decanted into 

 another llask and further cooled by contact with solid carbonic 

 acid and ether. The whole (3 or 4 ounces) solidified ; and the 

 temperature remained perfectly constant at —22° to —23° (say 

 —23°) until the last drop had frozen at —23°; it then sank 

 rapidly. 



If the vessel be continually shaken during crystallization, the 

 form of the crystals may be very clearly seen. While the anhy- 

 drous salt crystallizes in the well-known cubes, the bihydrate 

 separates as iridescent scales, and the body we are now consi- 

 dering solidifies in acicular bundles radiating from nuclei, and 

 much resembling in appearance the supersaturated solution of 

 sulphate of sodium when solidifying. It is, however, of a more 

 than pearly whiteness, and finally of complete opacity. 



§ 13. To see whether we have here indeed a definite hydrate, 

 the whole was remeltcd by the warmth of the hand and succes- 

 sively partially refrozen under continual agitation. Each mother- 



