DETERMINATION OF THE FREEZING POINT. 765 



too cold and the ice has been brought into the room out 

 of the open ah\ If the water containing the melting 

 pieces of ice be briskly stirred, the top of the mercury 

 will remain stationary as long as there is a very small 

 portion of ice left in the vessel. But if the ice or snow 

 be obtained from the open air during a severe frost, the 

 mercury of the thermometer falls at first to the tem- 

 perature of the mass of ice, which is below the freezing 

 point of water, and as the ice becomes gradually heated 

 by contact with the warm air of the room, the mercury 

 slowly rises and becomes finally stationary when the ice 

 begins to melt. Properly speaking, therefore, the tem- 

 perature which is determined by the experiment is that 

 at which ice melts, not that at which water freezes. It 

 will, however, be seen farther on that these two tem- 

 peratures are identical. 



If the scale of a thermometer is correct, the mercury 

 should remain stationary at 0, when it is immersed in 

 melting ice ; if this is not the case, the difference should 

 be noted, and must hereafter be applied as a correction 

 when the thermometer is used for indicating tempera- 

 tures. 



For the determination of the freezing point it is best and most 

 convenient to use snow ; if it cannot be had, and the ice obtained is 

 in large lumps, it should be broken into very small pieces with a 

 strong knife, which is used as if it were intended to cut thin slices 

 off the mass, or it may be crushed with the hammer. This is best 

 clone by putting the ice upon a board, placing another board on the 

 ice, and striking with the hammer on the upper board ; still, the 

 upper board will not altogether prevent many pieces from flying 

 about. This may be almost completely prevented by loosely wrap- 

 ping the ice to be crushed in a piece of strong cloth ; but the cloth 

 is easily torn by the sharp edges of the broken pieces. 



For determining the boiling point a flask with a long 



