28 



SCIENTIFIC AMUSEMENTS. 



specially constructed little machines with an agitator worked by a 



handle (fig. 14) are considerably more convenient. 



Expt. 23. To Freeze Mercury. Many substances can be used 



instead of salt to produce 

 "freezing mixtures" by 

 mingling them with snow 

 or crushed ice. One of 

 the most powerful freez- 

 ing mixtures of this class 

 is obtained by mixing to- 

 gether 2 parts of snow 

 and 3 of crystallised chlo- 

 ride of calcium; this is 

 sufficiently powerful in 

 its action to freeze mer- 

 cury, if the metal and all 

 the materials have been 

 previously somewhat 

 cooled down by placing 

 them in vessels chilled 

 by contact with ice. A 

 depression in temperature 

 of upwards of 40 centi- 

 Fig. 14. Household Ice Machine. gra( } e i s effected by this 



mixture, whereas a mixture of snow and salt (about 5 parts of 

 the first to 2 of the second) only causes a lowering of temperature 

 of about 20. 



Expt. 24. To Freeze Water without using Ice. Many 

 chemical substances are known which when made to dissolve in 

 water or other fluids will generate a considerable degree of cold. 

 Saltpetre (potassium nitrate) is such a substance, and has been 

 long used in India and other hot countries as a means of cooling 

 beverages, &c., the saltpetre and water being placed in a pail and 

 the wine or other liquid to be cooled being immersed therein in 

 bottles or other convenient receptacles. A better "frigorific 

 agent" is the salt termed nitrate of ammonium; by taking a 

 quantity of this in powder and stirring it with about its own 

 weight of water, sufficient cold is often obtained to freeze water, 

 especially if the salt and the water used have been all previously 

 cooled down as much as possible. Instead of nitrate of ammonium 

 a mixture of powdered saltpetre and salammoniac (chloride of 

 ammonium) may be used ; a lowering of temperature of upwards 

 of 20 centigrade is produced in either case. A still more effective 

 mixture, producing a depression of temperature of about 30 



