GALVANISM. 



357 



called decomposition from elective affinity. The 

 effect of heat, likewise, in promoting combination or 

 decomposition, may often depend on its exciting 

 electrical energy ; and the elevation of temperature 

 and production of light, so frequently attending che- 

 mical action, may depend on the clianges attending 

 the electrical states, since such changes are accom- 

 panied with the evolution of heat and light. The 

 agency of the galvanic apparatus, then, in producing 

 decomposition, it is conceived on this hypothesis, is, 

 that the two wires placed in contact with the com- 

 pound, are, in states of electricity, more intensely 

 exalted than the natural states of the two ingre- 

 dients ; hence the attraction of these two highly 

 electrified points overcomes that subsisting between 

 these ingredients : they are separated, and immedi- 

 ately drawn to the respective poles, the positive 

 constituent to the negative wire, and the ingredient 

 which is naturally negative, to the positive wire. A 

 number of facts are Drought forward in support of 

 these views. Thus, when dry acids, such as the 

 oxalic and boracic, are touched with an insulated 

 plate of copper, the electric equilibrium is imme- 

 diately disturbed ; the acids are found, after the con- 

 tact, to be in the negative state of electricity, and 

 the metal to be positive. Here then it was sup- 

 posed, that their natural states are manifested, such 

 as they are, inherently, at all times. Again, when 

 the same plate is applied to earthy and alkaline sub- 

 stances, the opposite appearance is presented ; the 

 metal becomes negative, and the latter bodies posi- 

 tive. And lastly, when acids are brought in con- 

 tact directly with earthy and alkaline substances, 

 the same relative states are exhibited the former 

 become negative, and the latter positive. 



To these speculations, however, it has been 

 objected, that there is not the slightest evidence that 

 bodies are naturally in particular electric states. 

 That they become either positive or negative when 

 submitted to certain operations, is no proof that they 

 exist originally in one condition more than another. 

 Since the tendency, also, always is to an electric 

 equilibrium, if two substances were naturally in 

 opposite states, and were, by the electric attraction, 

 brought into combination, as soon as they united, 

 the opposition of states would cease, an equilibrium 

 would result, and no attractive force would remain 

 to keep them in union. It has also been shown, in 

 opposition to this hypothesis, that bodies in opposite 

 states of electricity, do not combine when presented 

 to each other, and that bodies in similar states com- 

 bine with as much force as if in dissimilar states. 

 The theory, therefore, does not yet stand on so firm 

 a basis as to induce chemists to abandon the nomen- 

 clature they have hitherto employed, and cease to 

 regard affinity as a distinct species of attraction. 

 But at the same time it must be admitted, that the 

 electro-chemical theory is founded on extensive 

 observation and numerous facts, and has proved, in 

 the hands of its distinguished author, a safe guide to 

 some of the most famous discoveries ever made in 

 chemistry. Regarding all compounds as constituted 

 of oppositely electrical elements, Sir H. Davy con- 

 ceived that none of them should resist decom- 

 position, if exposed to a battery of sufficient inten- 

 sity ; and he accordingly subjected to galvanic 

 action substances which till then had been regarded 

 as simple, expecting that if they were compound 

 they would be resolved into their elements. The 

 alkalies and earths were in this manner succes- 

 sively decomposed ; a substance, with the aspect and 

 properties of a metal, appeared at the negative pole, 

 while oxygen gas was disengaged at the positive sur- 

 face. Another instance of the successful application 

 of these views is seen in the attempts of Sir H. Davy 



to protect the copper sheathings of shins from corro- 

 sion. It is well known that the copper sheathing of 

 vessels oxidizes very rapidly in sea water, and, con- 

 sequently, wastes with such rapidity as to require 

 frequent renewal. Sir H. Davy observed that the 

 copper derived its oxygen from atmospheric air dis- 

 solved in the water, and that the oxide of copper 

 then took muriatic acid from the soda and magnesia, 

 forming with it a sub-muriate of the oxide of copper. 

 Now, if the copper did not oxidize, it could not com- 

 bine with muriatic acid ; and, according to Sir II. 

 Davy, it only combines with oxygen, because, by 

 contact with that body, it is rendered positively 

 electrical. If, therefore, the copper could by any 

 means be made negative, then the copper and oxygen 

 would have no tendency to unite. The object, then 

 was to render copper permanently negative. Now 

 tin's is done by bringing copper in contact with zim-. 

 or iron ; for the former then becomes negative, and 

 the latter positive. Acting on this reasoning, it was 

 found that the oxidation of the copper might be com 

 pletely prevented. A piece of zinc as large as a pea, 

 or the head of a small round nail, was found fully 

 adequate to preserve forty or fifty square inches of 

 copper ; and this wherever it was placed, or under 

 whatever form it was used. Every side and every 

 surface of the copper remained bright, whilst the 

 iron or the zinc was slowly corroded. Unhappily for 

 the application of this principle in practice, it is found 

 that unless a certain degree of corrosion takes place 

 in the copper, its surface becomes foul, from the ad- 

 hesion of seaweeds and shellfish. It is possible, how- 

 ever, that, by duly adjusting the proportion of iron 

 and copper, a certain degree of corrosion may be al- 

 lowed to occur, sufficient to prevent the adhesion of 

 foreign bodies, and yet materially retarding the waste 

 of the copper. 



A more successful application of these principles 

 has been suggested by Mr Pepys, which is to pre 

 serve iron or steel instruments from rust by contact 

 with a piece of zinc. The iron or steel is thereby 

 rendered negative, while the zinc, being positive, 

 oxidizes with increased rapidity. 



It is to the electro chemical theory, also, that 

 chemistry owes the most philosophical arrangement 

 of which it appears capable. By it bodies are divided 

 into groups, accordingly as their natural electric ener- 

 gies are the same or different. The electric energies 

 are ascertained by exposing compounds to the action 

 of a galvanic battery, and observing the pole at which 

 the elements appear. Those that collect around the 

 positive pole are said to have a negative electric 

 energy; and those are considered positive electrics 

 which are attracted towards the negative pole. The 

 following list, showing the electric energy of the dif- 

 ferent elementary substances in relation to each 

 other, is taken from Berzelius's System of Chemistry. 

 They are given by their author as an approximation 

 to their true order, rather than as rigidly exact. All 

 bodies enumerated in the first row are negative to 

 those of the second. In the first column, each sub- 

 stance is negative to those below it ; and in the 

 second, each element is positive, compared with the 

 subsequent ones. 



i. 



Negative Electrics 

 Oxygen. 

 Sulphur. 

 Nitrogen. 

 Chlorine. 

 Iodine. 

 Fluorine. 

 Phonphnrus. 

 Selenium. 

 Arsenic. 

 Chromium 

 Molybdenum. 

 Tungsten. 



2. 



Positlie Etectrict 

 Potassium. 

 Sodium. 

 Lithium. 

 Barium. 

 Strontium. 

 Calcium. 

 Magnesium. 

 Beryllium. 

 Yttrium. 

 Aluminum. 

 Zirconium. 



