Page:The New International Encyclopædia 1st ed. v. 06.djvu/873

ELECTRICITY.

equivalents of a few different substances are given in the following table:

Assuming the truth of Faraday’s first law, a convenient method is offered for comparing the current-streneth of different currents, viz. piss the currents in turn through the same electrolyte and weigh the quantities liberated in the same time. The current-strengths are in the ratio of these masses. This is the principle of the so- called silver or copper ‘voltameters.’

Faraday pictured the phenomena of electrolysis as being due to the passage in the electrolyte of two streams of particles—one set. charged posi- tively going toward the cathode. the other charged negatively going toward the anode. These earriers of charges he called ‘ions’; and he thought of them as giving up their charges to the cathode and anode. and then being liberated from the liquid, as gases or solid deposits. or as then taking part in chemical reactions, and thus liberating other substances. It is easily seen that, if Faraday’s laws are exact. all ions of the same substance, e.g. hydrogen. must carry the same charge. in whatever electrolyte they appear: and further, that the charge on any ion is propor- tinnal to its valence. (Thus. if the charge on a hydrogen ion is e, that on a copper ion must be 2e, because the valence of copper is twice that of hydrogen.)

The modern theory of electrolysis is one of the most important branches of physical chemistry. Only its essential ideas need be mentioned here. As it is known by evidence from many sides that an electrolyte consists of a pure liquid in which a salt or acid is dissolved and dissociated into parts, it is natural to identify the ions with these fragments of the molecules of the salt or acid. The theory at present accepted is that by the act of solution in a liquid some substances are dissociated, at least in part: e.g. sulphuric acid in water: that is. some of the molecules of the dissolved substances break up into smaller parts. It is not thought that the parts of a molecule once dissociated remain separated, but that there is a state of dissociation and recom- bination going on as the molecules and their parts move about through the liquid. and at any in- stant a definite proportion of molecules are dis- sociated. Tt is natural to think that if a mole- eule is broken into parts. which are then sepa- rated. they will be electrically charged, one por- tien positively. the other negatively. (The charges on any part would then vary as its ya- Tenee.) Tf now a difference of potential is pro- duced in the liquid by inserting the anode and cathode. all the positively charged parts will. during their intervals of dissociation, be urged toward the cathode. while those nevatively charged will move toward the anode. (It is pos- sible that these dissociated parts do not move freely by themselves. but drag with them definite masses of the solvent.) On this theory, which accounts perfectly for the observed phenomena of electrolysis, the ions are charged fragments of molecules. When they get to the anode or cathode they give up their charges, and combine with other jens, thus forming molecules which are liberated, See ;, ,

There is reason fer believing that a pure would be a perfect insulator; but all gases can be made conductors in many ways, in all of which there is evidence that the molecules of the gas have been broken in- to parts— i.e. ‘ionized’; although the ions in a gas are by no means the same as in a liquid electro- lyte. Some of the methods for ionizing a gas are the following: (1) Application of high tempera- ture, eg. the gases rising from a Bunsen flame are conductors. (2) Passage of a spark in the neighborhood. (3) Exposure to Röntgen rays or to rays from a radio-active substance, such as uranium (q.v.). (4) Exposure to ‘cathode rays’ (see helow). (5) Exposure to ‘ultra-violet light.’ {See ) One most important fact which seems to be established is that the negative ions of all gases, however these ions are produced. are identical. It can be proved also that the negative ions have less mass than the positive ones. For a full discussion of the recent work on this sub- ject. reference must be made to -J. J. Thomson’s The Discharge of Electricity’ Through Gases (London, 1902) and to publications by Professor Thomson and his students in the Philosophical Transactions (London, current).

One of the most interesting illustrations of the passage of electricity through a gas is af- forded when the gas is inclosed at low pressure in a glass tube, into which enter two metal wires serving as anode and cathode. When the gas is at a pressure of a few centimeters of mercury. and a discharge of electricity through the tube is produced by an induction coil or by a number of cells, the gas near the anode is apparently ar- ranged in colored striations across the tube. while near the cathode there is a dark space and over the cathode itself there is a velvety glow. (The light emitted from the tube is characteristic of the gas inside. See Spectroscopy.) As the tube is exhausted further. so that the pressure of the eas decreases, the dark space round the cathode extends through the tube. Finally, if the exhaustion is continued, the character of the phenomenon is changed by the gradual appear- ance of the ‘cathode rays.’ These are streams of minute particles of matter negatively charged. which leave the cathode perpendicularly and pro- ceed in straight lines through the gas in the tube. These cathode rays are themselves invisi- ble, but they produce luminous effects where they pass through the gas and where they strike the walls of the tube: at this last place they produce thermal effects also. They produce mechanical motion if they strike any small movable object m the tube. Their path can be deflected by a magnet, proving that they are negatively charged particles moving with great speed—about 25,000 miles per second. If at the end of the tube on which the cathode rays strike there is a small opening covered with a thin sheet of aluminium. similar rays are observed outside the tube. These were first observed by Lenard and are called ‘Lenard rays” Cathode rays affect photographic