We cannot conceive that the particle _A_ excites the particle _B_ next it, and so on through the series between the two shores; but regarding the channel as one large drop, charged with the electric principle as we know it to be, it is excited by undulation or tremor throughout its width, and we have an equivalent of oxygen thrown off on one side of the line, and an exact equivalent of hydrogen at the other, the electro-chemical influence being exerted only where the current or motion is transferred from one medium to another.[145] The imperfect character of this view is freely admitted; no other, consistent with known facts, presents itself by which the effect can be explained. The fact stands as a truth; the hypothesis by which it is attempted to be interpreted is open to doubt, and it is opposed to some favourite theories. Before we pass to the consideration of the other sources of electricity, it is important we should understand that no chemical or physical change, however slight it may be, can occur without the development of electrical power. If we dissolve a salt in water, if we mix two fluids together, if we condense a gas, or convert a fluid into vapour, electricity is disturbed, and may be made manifest to our senses.[146] It has been shown that this power may be excited by friction (machine electricity) and by chemical action (voltaic electricity, galvanism); it now remains to speak of the electricity developed by heat (thermo-electricity), the electricity exhibited under nervous excitement by the gymnotus and torpedo (animal electricity); magnetism and its phenomena being reserved for a separate consideration. If a bar of metal is warmed at one end and kept cool at the other, an electrical current circulates through the bar, and may be carried off by connection with any good conductor, and shown to exhibit the properties of ordinary electricity. The metals best suited for showing the effects of thermo-electricity appear to be bismuth and antimony. By binding two bars of these metals together at one end, and connecting the other ends with a galvanometer, it will be discovered that an electric current passes off through the instrument by the slightest variation of temperature. Merely clasping the two metals, where bound together, with the finger and thumb, is sufficient to exhibit the phenomenon. By a series of such arrangements,--which form what have been called thermo-electric multipliers,--we obtain the most delicate measurers of heat with which philosophers are acquainted, by the aid of which Melloni has been enabled to pursue his beautiful researches on radiant caloric. That this electricity is identical with the other forms has been proved by employing the current thus excited for the purpose of producing chemical decomposition, magnetism, and electric light.[147]