RETARDATION OF SOLIDIFICATION. 789 



over the ice, and the bulb of the water-hammer, which is clamped in 

 the position shown in the figure, is immersed in the mixture. After 

 10 or 15 minutes the water is cooled below 0, but is still liquid ; 

 the apparatus is then gently raised out of the mixture, and cautiously 

 wiped with a cloth so as to avoid moving the vessel containing the 

 water. It is then vigorously shaken, and the water will be con- 

 verted into ice. 



The crude rock salt of commerce is more economical for freezing 

 mixtures than table salt. 



In order to prove by means of a thermometer that the temperature 

 of the water really falls below its freezing point, the instrument 

 must be placed in the liquid before it is cooled. To insert the 

 thermometer into the liquid after it has been cooled to a tempera- 

 ture lower than its point of solidification would be quite useless, for 

 the immersion of the instrument causes an amount of agitation 

 which is sufficient to produce solidification of the liquid, and this is 

 always accompanied by a disengagement of heat and a rise of tem- 

 perature of the whole liquid up to its ordinary freezing point, which 

 would therefore be the temperature indicated by the instrument. 

 The temperature to which the water is cooled may be observed, if a 

 glass flask is nearly filled with water and then closed by a per- 

 forated cork, through which a thermometer passes. The bottle 

 may then be placed during the winter in the open air, and as the 

 liquidity is longer continued in a closed vessel than in one which is 

 quite open, the phenomenon is likely to be completely observed on 

 the first occurrence of frosty weather. 



The retardation of solidification and the disengage- 

 ment of heat at the moment when solidification takes 

 place may be well observed in the case of sodium 

 hyposulphite, a white crystalline salt largely used in 

 photography. The salt melts at 57, and when care- 

 fully cooled it remains liquid at the ordinary tempera- 

 ture of the atmosphere. It may then be made to 

 solidify by a rapid agitation, or still better, by sprink- 

 ling over the surface of the liquid a few grains of the 

 solid salt. The rise of temperature resulting from the 

 solidification is such as to be distinctly felt by the 

 hand. 



