Force and Form. 1047 



which raises the sand on the windward side of the hill and blows it 

 over the crest where it settles in the still air under shelter of the lee 

 side. The hill thus gradually proceeds across the country, the wind 

 following it up. Similar action is caused by water currents, that change 

 channels, thereby changing their own direction. 



Analogous to this is the action of the molecular energies, such as 

 magnetism, chemism, nervous energy, mentality, &c. All of them 

 tend to work alterations in the instruments upon which the} r operate 

 and thus to cause modifications in the phenomena or functions resulting 

 from their operations. 



' ' When a steel or iron bar is powerfully magnetized, it grows a little 

 longer than before, and since its volume is the same as before it at the 

 same time contracts in thickness. Joule found an iron bar to increase 

 by i~-^z f i ts length when magnetized to its maximum. This phe- 

 nomenon is believed to be due to the magnetization of the individual 

 particles, which, when magnetized, tend to set themselves parallel to 

 the length of the bar. This supposition is confirmed by the observa- 

 tion of Page, that at the moment when a bar is magnetized or demag- 

 netized, a faint metallic clink is heard in the bar. Sir W. .Grove 

 showed that when a tube containing water rendered muddy by stirring 

 up in it finely divided magnetic oxide of iron was magnetized, the 

 liquid became clearer in the direction .of magnetization, the particles 

 apparently setting themselves end-on, and allowing more light to pass 

 between them. A twisted iron wire tends to untwist itself when mag- 

 netized. A piece of iron when powerfully magnetized and demagnet- 

 ized in rapid succession grows hot, as if magnetization were accom- 

 panied by internal friction. " l 



The molecular structure of metals controls their conductivity of heat 

 and electricity, the resistance being much greater in some metals than 

 in others, but practically the same for heat that it is for electricity. 

 When ;i current of electricity is sent through a poor conductor, the re- 

 sistance turns the electricity to heat. As Prof. Tyndall pictures it, the 

 current exhausts itself by knocking against the obstructing molecules 

 in struggling to get past them. In the case of the good conductor the 

 molecules lie in positions to allow of easy movement to the ether, and 

 so the current gets b}' without disturbing them or heating them. It is 

 settled that the conduction of electricity in a perfect conductor is the 

 same as that of light in ether; viz., 186,000 miles per second. (See 

 page 345). 



Attention was called tit the beginning of chapter 37 to the influence 

 of the form of the molecules of bodies on their chemical reactions. 

 This is without doubt to be attributed to the directive influence of form 



1 S. P. Thompson, Electricity and Magnetism, p. 93. 



