Propagation of the Electric Force. 151 



The simplest and clearest course, and that most reconcileable with the laws of 

 statical electricity, seems to me to be : — to consider that no electrical development 

 is caused by the union of an alkali with an acid, (the electricity being thereby dis- 

 guised,) but that, at the instant before the union takes place, the particles of the 

 alkali and of the acid, being in opposite electrical states, affect their surrounding 

 particles by induction, causing thereby a feeble current of electricity to circulate 

 from the acid through the galvanometer to the alkali, which supposition is borne 

 out by the fact that a dry acid and alkali, when in contact, show opposite electrical 

 states. 



The same arguments apply equally well with regard to thermo-electricity. 

 The contact of two metals produces in them opposite electrical states. Their 

 chemical union in an Isolated vessel gives no electrical development ; thus a 

 " solid amalgam of bismuth and lead become liquid when mixed together, with- 

 out producing any electrical effect." * Again, " a thin plate of zinc placed 

 upon a surface of mercury, and separated by an insulating body, is found to be 

 positive, the mercury negative ; but when kept together a sufficiently long time to 

 amalgamate, the compound gives no signs of electricity."* 



These experiments explain why the contact of the two extremities of metallic 

 wires, constituting a closed circuit, should, as the potash and nitric acid just 

 mentioned, produce an induced electric current. That the electric states of dif- 

 ferent metals in contact, when excited by heat, do not follow the law of their 

 natural electrical states, and change on increase of temperature, is no argument 

 against the explanation I have given, for upon what this change in the electrical 

 excitation produced by heat depends, whether upon a peculiar arrangement of 

 the crystalline parts of the metal, or of their compound elementary particles, 

 we are as yet perfectly ignorant. 



That the same general law of the contact of metals and of fluids applies 

 equally (although in an inferior degree, owing to their want of conducting 

 power) to the contact of the gases, may be shown by the experiment of Dr. 

 Faraday (Sixth Series) of the union of hydrogen and oxygen by a plate of pla- 

 tinum ; the electrical force, which circulates by the Interposed platinum plate, 

 facilitating the union of the two gases.f 



* Phil. Trans., Bakerian Lecture, 1807. 



■\ Aqueous solutions of different gases, when brought into contact, have been found to produce 

 electrical currents. 



