it came to another particle, and transferred its power and motion, making that other particle active.  Now, though the particle of air travelled over a visible and occasionally a large space, whilst the particle of the electrolyte moved over an exceedingly small one; though the air particle might be oxygen, nitrogen, or hydrogen, receiving its charge from force of high intensity, whilst the electrolytic particle of hydrogen had a natural aptness to receive the positive condition with extreme facility; though the air particle might be charged with very little electricity at a very high intensity by one process, whilst the hydrogen particle might be charged with much electricity at a very low intensity by another process; these are not differences of kind, as relates to the final discharging action of these particles, but only of degree; not essential differences which make things unlike, but such differences as give to things, similar in their nature, that great variety which fits them for their office in the system of the universe.

1624.  So when a particle of air, or of dust in it, electrified at a negative point, moves on through the influence of the inductive forces (1572.) to the next positive surface, and after discharge passes away, it seems to me to represent exactly that particle of oxygen which, having been rendered negative in the electrolyte, is urged by the same disposition of inductive forces, and going to the positive platina electrode, is there discharged, and then passes away, as the air or dust did before it.

1625. Heat is another direct effect of the current upon substances in which it occurs, and it becomes a very important question, as to the relation of the electric and heating forces, whether the latter is always definite in amount[A].  There are many cases, even amongst bodies which conduct without change, that at present are irreconcileable with the assumption that it is[B]; but there are also many which indicate that, when proper limitations are applied, the heat produced is definite.  Harris has shown this for a given length of current in a metallic wire, using common electricity[C]; and De la Rive has proved the same point for voltaic electricity by his beautiful application of Breguet’s thermometer[D].

  [A] See De la Rive’s Researches, Bib.  Universelle, 1829, xl. p. 40.

  [B] Amongst others, Davy, Philosophical Transactions, 1821, p. 438. 
  Pelletier’s important results, Annales de Chimie, 1834, lvi. p. 371.
  and Becquerel’s non-heating current, Bib.  Universelle, 1835, lx. 218.

  [C] Philosophical Transactions, 1824, pp. 225. 228.

  [D] Annales de Chimie, 1836, lxii. 177.

1626.  When the production of heat is observed in electrolytes under decomposition, the results are still more complicated.  But important steps have been taken in the investigation of this branch of the subject by De la Rive[A] and others; and it is more than probable that, when the right limitations are applied, constant and definite results will here also be obtained.