56 SCIENCE PRIMERS. [MATERIAL 



fluid state ; but, even down to the freezing-point, some 

 water will remain in the gaseous state and will fill that 

 part of the flask which is unoccupied by the fluid water. 

 At blood-heat (98) the gaseous water weighs only 

 about a grain, though it still occupies 100 cubic inches; 

 at the ordinary temperature of the air it weighs not 

 more than ^-rd of a grain ; while, at the freezing-point, 

 its weight is only Jth of a grain. But inasmuch as 

 there is less and less actual weight of water in the 

 same volume of gaseotis water as the temperature 

 falls, it follows that the density, or specific gravity, of 

 the gaseous water must be less the lower the tem- 

 perature. Moreover, while, at the boiling-point, 

 gaseous water or steam resists compression with 

 exactly the same force as air does, the lower the 

 temperature the more easily compressible is the 

 gaseous water. 



Suppose an elastic bag were to be tied on to the 

 nozzle of a kettle full of boiling water. If the bag 

 were kept as hot as the boiling water it would become 

 fully distended, and maintain its shape in spite of 

 the pressure of the air upon all sides of it. If the bag 

 were taken away it would retain its shape so long as 

 it was kept as hot as boiling water ; but, if it were 

 allowed to cool, it would gradually become flattened 

 by the outside air squeezing up the less and less resist- 

 ing gaseous water of the lower temperatures. Hence, 

 when the stopped flask has been allowed to cool, the 

 air rushes in with" great violence if it is opened. 



38. The Evaporation of Water at ordinary 

 Temperatures. 



If some water is poured into a saucer and is 



