34 CHEMISTRY FOR AGRICULTURAL STUDENTS 



relative densities by means of the hydrometer. This is an 

 instrument constructed to float upright in a liquid, and which 

 records the relative density of the liquid by the depth to which it 

 sinks, the stem being graduated in " degrees of specific gravity," 

 water being taken as looo. The liquid must be at 15° C. 

 Compare the relative density of the salt solution as determined 

 by the hydrometer with that determined by the previous 

 methods. 



Other Properties of Water. — Water has a great heat capacity. If 

 beakers of water and mercury be placed side by side in a bath of hot 

 water, the mercury will become hot much sooner than the water, and, when 

 both are hot, the water will remain hot much longer. Water has alsp a 

 greater heat capacity than the constituents of rocks and soil. If into each 

 of two beakers, the one containing a pound of cold water and the other a 

 pound of sand, a pound of boiling water be poured, and each stirred with 

 a thermometer, the temperature of the mixture of sand and water will be 

 found to be the higher, the heat capacity of the sand being less than the 

 water, and requiring less heat to heat it than the cold water. The heat 

 that would raise i lb. of water i°C. would raise 5 lbs. of sand i°, and would 

 raise i lb. of sand 5°. The relative or specific heat capacity of sand is 

 therefore .2 ( water =i). Water is taken as the standard, as it has the 

 greatest heat capacity of all ordinary solid or liquid substances. It is 

 partly for this reason that Great Britain, being surrounded by water, has so 

 temperate a climate, because the sea absorbs more heat in summer and 

 gives up more heat in winter than land. For the same reason a dry, sandy 

 soil is "warmer" than a clay soil, because the latter retains so large a 

 quantity of water. 



When change of state occurs from a solid to a liquid, or a liquid to a gas, 

 heat is absorbed. Since heat is again produced when these changes of state 

 are reversed, it is spoken of as the latent heat of liquefaction, and the 

 latent heat of vaporisation. An example of the latter is seen in the 

 wet and dry bulb thermometers. The wet bulb thermometer will read the 

 lower, unless the air be saturated with water vapour, because the evapora- 

 tion from the surface is accompanied by absorption of heat. An example 

 of the former is the freezing mixture produced by mixing salt and 

 snow. The salt produces rapid melting of the snow, and heat is conse- 

 quently absorbed, the tem.perature falling to -21° C. The loss of heat by 

 the great evaporation from the sea during the summer is another cause of 

 the temperate character of the climate of Great Britain. For this reason 



