196 LECTURES. 



(24.) Porosity of liquids : 



Water and sulphuric acid ; water and alcoliol ; salt and water ; 

 water and gas. 



(25.) Porosity of gases : 



Air and vapor ; nitrogen and hydrogen. 



Some bodies are without pores of the third order. Examples : 

 glass, crystals, &c. ; but these can be compressed, and, therefore, 

 have pores of an inferior order. 



(26.) Compressibility of solids, by mechanical means : 



Of iron in casting; of brass for delicate machines; of wood, so as to 

 sink in water; of cork in neck of bottles lowered into the deep sea. 



(27.) Of water and other liquids. 

 Apparatus of Perkins. Of CErsted. See Elasticity. 

 (28.) Compressibility of air and all gases. 



Experiment with air in a tube submitted to great pressure under 

 water. 



DILATABILITY. 



(29.) All bodies change their volume with a change of temperature. 

 Examples : air expanded by heat ; also water ; bar of metal 

 lengthened ; Saxton's apparatus employed. 

 Preliminary notions of heat. 

 General description of the thermometer, 

 Dilatability by the removal of pressure. 

 Examples in the case of air ; water ; solids. 



(30.) By mechanical exertion. 



Examples : When India rubber is stretched, its density is said to 

 be slightly lessened. Also, when wire is drawn in the direction of its 

 length, the same effect is produced. 



MOBILITY. 



"Tol.) The property by which a body is capable of a change of place. 



Motion is better illustrated than defined. The following definition, 

 "however, is sometimes given : 



Motion is the rectilinear change of distance between two points. — 

 \Dr. Young. 1 



^ According to this definition, if there were but one point in the 

 universe, there could be no measurable motion. 



(32.) Rest is permanency in the same place. 



It is only apparent ; all bodies are really in motion, expanding and 

 contractino- with the constant change of temperature — moved by every 

 sound • in motion with the earth on its axis and in its orbit. 



Rest and motion of two kinds, absolute and relative. 



Illustrations : 



Direction of motion ; continued and reciprocating ; rectilinear and 



circulo.r. 



The line described is sometimes called the trajectory. 



