86 GENERAL SCIENCE 



these intermolecular spaces (pores), and that the degree of 

 this motion and the consequent number of collisions among 

 the molecules is the cause of the varying temperatures of 

 bodies. It is believed that the distinctions between solids, 

 liquids, and gases lies largely in the different intensities of 

 the attractions (pulls) exerted between the molecules by the 

 intermolecular forces of cohesion and adhesion. In solids 

 the attractive forces dominate, and the solid retains a form 

 of its Own. In gases the moving molecules drive one another 

 apart, and gaseous matter must be confined. Left unre- 

 stricted, gases would expand indefinitely filling all space 

 open to them. Liquids are an intermediate condition be- 

 tween solids and gases, and possess enough of freedom in the 

 motion of their molecules to assume the form of a containing 

 vessel. Molecules escape from their upper surfaces into the 

 atmosphere more or less at all times when left uncovered. 

 The rate of this evaporation varies with the kind of liquid, 

 its temperature, and the capacity of the atmosphere for 

 holding that particular vapor. Many liquids are said to 

 be volatile because they must be kept in stoppered bottles 

 or other containers to prevent excessive waste due to 

 evaporation. 



Water is boiling when it is changing to a vapor within the 

 liquid mass. For pure water under one atmosphere of pres- 

 sure, .i.e., when the barometer reading is thirty inches (76 

 cm.), the boiling temperature is 100 C. (212 F.). When 

 water is confined in a closed vessel, as in the boiler of a loco- 

 motive, and the gaseous (steam) pressure upon it becomes 

 more and more, the water must be heated hotter and hotter to 

 keep it boiling as the pressure upon it increases. Under two 

 atmospheres (thirty pounds per square inch) the boiling 

 point becomes about 120 C., and at fifteen atmospheres 

 becomes about 200 C. If at any such high temperature the 

 pressure on the water surface be suddenly released, the heat 



