80 



JUNIOR GRADE SCIENCE 



the pressure upon it is changed, being raised if the pressure is greater, 

 and lowered if the pressure is less. When the water boils, the tempera- 

 ture of the steam is the same as that of the water, and remains so as 

 long as there is any water left. The lower fixed temperature is referred 

 to as the " Freezing Point of Water," and the higher as its " Boiling 

 Point." 



Marking the freezing point. For this purpose an arrangement 

 like that shown in Fig. 64 is very suitable. The funnel is filled with 



FIG. 64. Thermometer in ice for the 

 observation of freezing point. 



FIG. 65. Flask fitted for the deter- 

 mination of boiling points. 



pounded ice, which before powdering has been washed carefully : or 

 snow may, if more convenient, be used. The glass dish catches the 

 water which is formed from the melting of the ice or snow. A hole 

 is made in the pounded ice by thrusting in a pencil or glass tube about 

 the size of the thermometer, and into this hole the thermometer is 

 put and is so supported that the whole of the mercury is surrounded 

 by the ice or snow. The arrangement is left for about ten or fifteen 

 minutes, until it is quite certain that the tube and mercury are at the 

 same temperature as the melting ice. When this, is so the tube is raised 

 until the mercury is just above the ice, and a fine scratch is made with 

 a three-cornered file on the tube at the level of the mercury. 



