A very simple means may be adopted of showing what is thought to be one of the many evidences in favour of two electricities. If the wire carrying the current flowing from the machine, is passed over paper covered with nitrate of silver, it produces no change upon it; but if the wire which conveys the current to the instrument, when it is excited, is passed over the same paper, the silver salt is decomposed.[139] We may, however, explain this result in a satisfactory manner, upon the hypothesis that the decomposition is produced by the abstraction of electricity, rather than by any physical difference in the fluid itself. By frictional electricity we may produce curious molecular disturbances, and give rise to molecular re-arrangements, which have been called “electrical images,” in glass, in stone, and in the apparently less tractable metals: these images are rendered visible by the manner in which, according to their electrical states, some lines receive any particular powder, or vapour, which is repelled from other spaces. Many of the great natural phenomena, such as Lightning and Thunder, the Aurora Borealis, and Meteors, may be imitated in a curiously exact manner by the electrical machine and a few familiar arrangements.[140] Voltaic electricity, as the active force produced by chemical change is commonly called, in honour of the illustrious Volta, is now to be considered. It differs from frictional electricity in this:--the electricity developed by friction of the glass plate or cylinder of the electrical machine is a discharge with a sort of explosion. It is electricity suddenly liberated from the highest state of tension, whereas that which is generated by chemical action in the voltaic battery is a steady flowing current. We may compare one to the ignition of a mass of gunpowder at once, and the other to the slow burning of the same quantity spread out into a very prolonged train. There are numerous ways in which we may excite the phenomena of Voltaism, but in all of them the decomposition of one of the elements employed appears to be necessary. This is the case in the arrangements of batteries in which two dissimilar metals, zinc and copper, silver and platinum, or the like, is immersed in fluids; the zinc or the silver are gradually converted into soluble salts, which are dissolved, whilst the copper or platinum is protected from any action. The most simple manner of illustrating the development of this electricity is by placing a piece of silver on the tongue, and a piece of zinc or lead underneath it. No effect will be observed so long as the two metals are kept asunder, but when their edges are brought together, a slight tremulous sensation will pass through the tongue, a saline taste be distinguished by the palate, and if in the dark, light will be observed by the eye.