278 Prof. Norton on Molecular Physics. 



may be excited with two wires of the same metal, by heating 

 the end of one and bringing it in contact with the other ; and 

 that the direction of the current at the junction is from the cold 

 to the hot wire." In this case it is to be remarked that the hot 

 wire is in the act of cooling ; and hence there should be an elec- 

 tric movement toward its surface (p. 276), or from the cold to 

 the hot wire through the junction. 



Tyro -electric Crystals. — Tourmaline is the most conspicuous 

 crystal belonging to this class. "A prism of tourmaline has dif- 

 ferent secondary planes at its two extremities, or, as it is ex- 

 pressed, is hemihedrally modified." This peculiarity of crystal- 

 lization, which also obtains in the other pyro-electric crystals, 

 indicates that in the direction of the axis the molecules of the 

 crystal have different mechanical properties on opposite sides. 

 In this condition of things we may reasonably suppose that the 

 molecular atmospheres would expand unequally on opposite 

 sides, under the influence of the same amount of heat. If this 

 be admitted, we have a complete explanation of the electric phe- 

 nomena exhibited by the tourmaline when heated, in accordance 

 with the principles already laid down. Thus let a tourmaline 

 be heated regularly — that is, so that all points of its surface shall 

 receive equal increments of heat ; at all points of the surface the 

 unequal expansive action of the heat upon the two sides of the 

 molecular atmospheres in the axial direction will determine 

 their polarization and an attendant electric movement from the 

 positively polarized side of one molecule to the negatively polar- 

 ized side of the next. There should accordingly be opposite 

 electrical states manifested at the ends of the crystal. This state 

 of things should continue so long as the temperature is rising. 

 But it is to be observed that the effective polarization determined 

 in each molecule by the heat is weakened by the discharge that 

 takes place from one molecule to the next, and that from this 

 cause the signs of electrical excitement at the ends of the crystal 

 will be much feebler when the temperature becomes uniform 

 than they would otherwise be. Now if the heated tourmaline 

 be cooled regularly, the process that attended upon the heating 

 will be reversed, and the electrical states, or effective poles, will 

 be reversed. This reversal of the poles may occur soon after the 

 temperature begins to fall, because of the above-mentioned weak- 

 ening of the effective polarization, since this remaining polari- 

 zation, when the temperature becomes uniform, which is really 

 effective upon bodies placed near the ends of the tourmaline, 

 may be no greater than the opposite transient polarization that 

 may arise from a small loss of temperature. 



Mutual Attractions and Repulsions of Electric Currents. — We 

 have seen that in an electric current there is a flow or onward 



