According to Mr. Joule, the quantity of heat generated in a unit of time is proportional to the strength of the current, and when a galvanic current is employed in chemical analysis, the heat in the entire circuit generated in a unit of time is equal to the work expended in producing it, minus that employed in the analysis. In fact, a current of electricity cannot pass through a homogeneous conductor without generating heat in overcoming resistance, an effect proved by Mr. Joule to be proportional to the square of the force of the current, and the same in whatever direction the current may be flowing. Any other thermal action that can take place must depend upon the heterogeneousness of the circuit, and must be reversible with the current. For example, if a semicircle of bismuth be joined to a semicircle of antimony, an electric current in passing through it produces cold where it passes from the bismuth to the antimony by absorption, and heat where it passes from the antimony to the bismuth. The transit of the electricity from pole to pole is accompanied by light, and in consequence of the continuous current sparks occur every time the contact of the wires is either broken or renewed; but considerable intensity is requisite to enable the electricity to force its way through atmospheric air or gas. Both its length and colour are affected by the density of the medium through which it passes. If the medium be gradually rarefied the discharge increases from a spark to a luminous glow, differing in colour in different gases, but white in air. When very much attenuated a discharge may be made to pass across 6 or 7 feet of space, while in air of the ordinary density it will not pass through an inch. In rarefied gas it resembles the Aurora by its continuous flashes. When the battery is powerful the luminous effects are very brilliant.