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I have little doubt, however, that the explanations of these will be ultimately found in actions of a secondary character.

Note on Voltaic Batteries. 74. In the foregoing investigation I have had occasion to work very extensively with different voltaic arrangements, and have repeatedly ascertained their relative intensities by the mathematical theory of Ohm. It will not, therefore, I hope, be deemed out of place to subjoin a table, in which the intensities of the batteries which are most generally used, are inversely as the number of pairs which would be just requisite in order to overcome the resistance of water to electrolyzation. Platinum Amalgam.zinc

1 Mr. Grove's

Nitric acid Dilute sulp. acid 0.93
Copper
Sulphate
Amalgam. zinc

1 Prof. Daniell's

of oxide
Dilute sulphuric

1:54
acid

{ Nitric acid

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of copper

Constant Intensities.

1 Mr. Sturgeon's

Iron. Amalgamated zinc
Dilute sulphuric acid

3:33 Mr. Smee's

Platinized silver. Amalg. zinc ) 1 2 Dilute sulphuric acid

3:58 § Copper. Algamated zinc

1 Dilute sulphuric acid

5.40 75. Without entering particularly into the respective merits of these arrangements, I may observe that each of the first four may be used advantageously, according to the circumstances in which the experimenter is placed, or the particular experiments which he wishes to execute. The zinc-iron battery is somewhat inconvenient on account of local action on the iron; but then it presents great mechanical facilities in its construction. Mr. Smee's and Mr. Grove's are also very good arrangements; but the battery of Daniell is the best instrument for general use, and is, moreover, unquestionably the most economical. Broom Hill, Pendlebury near Manchester,

March 25th, 1841. P.8. In the above table of galvanic intensities, that of zinc-iron immersed in dilute sulphuric acid is somewhat overstated. Recent experiments convince me that when the iron is in its best condition it possesses the same powers as the platinized silver. I attributed the iron battery to Mr. Sturgeon, who constructed one of these excellent instruments early in 1839. It consisted of twelve castiron tubes, furnished with strips of amalgamated zinc. But I find that the experiments of this gentleman were not published as early as those of Mr. Roberts. Prof. Daniell (Phil. Trans. 1836, p. 114) observeil that iron is sometimes more efficient than platinum in voltaic association with amalgamated zinc.

J. P.J. August 11, 1841.

On the Electric Origin of the Heat of Combustion.

By J. P. JOULE, Esq.*

[Illustrated by Plate II.) 1. In the papers which I had some time ago the honour of communicating to the Royal Society. I related an investigation concerning the calorific effects of voltaic electricity, and stated my opinion with regard to the heat evolved by combustion and certain other chemical phenomena. In the present paper I intend to bring forward some experiments in confirmation of my theory, and to prove that the heat of combustion, terminating in the formation of an electrolyte, is the consequence of resistance to electric conductiou.

2. We have seen that wben those chemical actions which are not the sources of transmitted electricity are allowed for, the heat evolved from any part of the voltaic apparatus is the effect of the resistance which is presented by that part to the electric current; and that hence it necessarily follows, that the total voltaic heat generated by the action of any closed galvanic pair is proportional to the number of chemical equivalents which have been consumed in the act of propelling the current, and the intensity of the galvanic arrangement. Now, if it can be shown that the quantity of heat which is evolved by ordinary chemical combination is the same as the calculation founded on these facts would lead us to expect, no reasonable doubt can be entertained that it also is the product of resistance to electric conduction.

3. In studying the character of the heat of combustion, the first point was to determine the intensities of the affinities of different combustibles for oxygen. For this purpose I have, in accordance with the views which were first stated by Davy, and have since been adopted by the most eminent electricians, made use of the measure of those intensities which is afforded by the electric current.

4. I had not proceeded far before some curious phenomena were observed, which, though not very well understood, have long been knownt to electricians. I shall notice these first, because of their important bearing upon subsequent reasonings and conclusions.

5. I was working with an arrangement consisting of iron, platipized silver, and dilute sulphuric acid. The circuit was closed by a galvanometer, the coil of which consisted of 119 turns of thin silked copper wire, forming a rectangle, measuring one foot by six inches. T'he needle indicated a pretty constant deviation of 200, but on moving the platinized silver backwards and forwards the

• Read before the Literary and Philosophical Society of Manchester, November 2, 1841; and now communicated by the author.

+ In 1830, Mr. Sturgeon remarked that when two pieces of iron are placed in dilute muriatic acid, the agitation of one of them will make it operate as copper in the copper-zinc battery : also, that if two pieces of iron are immersed in succession in a solution of nitrous acid, the iron last immersed will act as copper in the copper-zinc battery.- Recent Experimental Researches, p. 46-49.' We shall hereafter see the trae cause of these phenomena.

needle advanced gradually to 40°, where it was kept for some time by continuing the agitation. As soon as the motion of the platinized silver was discontinued, the needle resumed its former position. Similar effects were produced by stirring the liquid, and thus causing it to impinge against the platinized silver.

6. I repeated the above experiment many times with similar results, but I found that whenever a large quantity of hydrogen had been evolved from the liquid by the action of the pair, or otherwise, the phenomena were not well produced. This circumstance convinced me that the effects were due to atmospheric air held in solution by the liquid, and that the displacement of a part of it by the hydrogen had occasioned their partial prevention. My opinion was confirmed by the following experiment.

7. I filled three quarters of the contents of a glass flask with dilute sulphuric acid, and then placed it over the flame of a spiritlamp until I judged that all the atmospheric air had been boiled out, I then removed the lamp, and immediately placed in the mouth of the flask a cork, through which a small piece of platioized silver and a stout iron wire had been passed. On connecting the metals with the galvanometer (5) its needle was deflected to 321 , and on shaking the flask very briskly it could not be made to advance further than 34o. This advance, slight as it is, was probably entirely occasioned by the air, which, notwithstanding my precautions, had found its way into the upper part of the flask.

8. The phenomena originated entirely from the platinized silver; and although a slight advance of the needle was sometimes produced by agitating the iron, it was not difficult to see that the real cause was the propulsion thereby occasioned of the aërated liquid against the negative* element, for when this was avoided no advance of the needle could be produced by agitating the positive metal.

9. I thought it probable that an increase of the intensity of the current would be produced by directing a stream of oxygen gas against the negative element. On making the experiment, I found that the needle advanced a few degrees, and that the same effect could be produced by a stream of hydrogen. There could be no doubt that the increase of intensity arose rather from the agitation of the liquid than from any specific action of the gases, and that this experiment was essentially the same as that described in (5).

10. I impregnated some dilute sulphuric acid with a very small quantity of oxygen, according to Thenard's process, and then immersed into it a plate of platinized silver and a rod of iron, both properly communicated with the galvanometer. The needle stood for the first few seconds at 68°; in three minutes it declined to 500; in five minutes more to 499, and in another five minutes to 481o. On agitating the platinized silver so as to bring it repeatedly in con

• To avoid misconception, it is perhaps as well to observe that I call those elements of the voltaic battery negative, which attract or combine with those bodies which are called "positively electrical,” or 6 cations."

tact with the yet undecomposed deutoxide of hydrogen, the needle advanced to above 60”. The same pair, immersed in common acid, would have deflected the needle no further than 29° or 30°.

11. The effect of the presence of oxygen at the negative element is well observed by making it, in water, the positive electrode of a voltaic battery. By this means oxygen is deposited on its surface, and is there ready to produce an extraordinary intensity. This deposit of oxygen is in fact the cause of the action of Ritter's secondary piles.

12. The following was also a very convenient method of showing the increase of intensity arising from the presence of oxygen. Some dilute sulphuric acid was agitated with chlorine until the former had taken up as much of the gas as it could. By pouring a solution of sulphate of oxide of silver into the liquid, I now precipilated chloride of silver, leaving sulphuric acid and oxygen in solution. When a pair, consisting of platinized silver and iron, was placed in the acid thus prepared, the galvanometer was permanently deflected 50°, and by agitating the platinized silver the needle advanced as far as 60°. When a piece of amalgamated zinc was substituted for the iron the permanent deflection of the needle was 65°, and by agitating the negative element as before, the needle advanced to 70°. Had the same pairs been immersed in a simple solution of sulphuric acid, the permanent deflections would bave been no greater than 30° and 63o.

13. Similar results were obtained with the solution of chlorine, as might have been anticipated from its strong affinity for hydrogen.

14. From the above experiments, we see that the agitation of the negative element is productive of an increase of intensity, simply because it is thereby brought into contact with bodies capable of combining with the hydrogen, which would otherwise have been evolved from it. When those bodies are present in considerable quantities, as in (10), (12), and (13), the intensity of the current is great, even though the pair be left quiet, because then the tive plate can collect them readily upon its surface. Again, by causing the current to encounter great resistance, the effects of agitation which we have noticed are proportionally increased, because then the number of particles required for neutralizing the hydrogen is less. Hence it is that when I have used a resistance of 500 or 600 yards of thin wire, I have frequently found the deviation of the needle (even when the pair was left quiet in a common solution of sulphuric acid) considerably greater than was due to that resistance. This is also the probable reason why De la Rive in one instance* found the intensity of the copper-zinc pile the same, whether charged with water or nitric acid,

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15. In the course of the preceding experiments I was forcibly struck with the very great intensity of the pairs at the moment of

Ann. de Chimie, 1836, part i, p. 179.

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their immersion, compared with that which they were able to maintain permanently. It appeared to me that the theories which had been put forth to explain the first effect of immersion, though seemingly plausible with regard to the zinc battery, were not at all equal to account for the same phenomenon as existing to a far greater extent when iron is used as a positive element.

16. A rod of iron and a small plate of platinized silver were immersed in a dilute solution of sulphuric acid. On connecting them with the galvanometer (6), the needle was permanently deflected 291. After a few preliminary trials to ascertain the proper point, I caused the needle to be maintained by a glass weight at 55°, beyond which it was free to travel. I then exposed the platinized silver to the air during one minute of time. On re-immersing it the needle sprang as far as 60°, and then immediately recoiled to its resting place at 55°, thus indicating a transitory current of about 57.

17. Onexposing the platinized silver for 5" only, the transitory current, ascertained in a manner similar to that just mentioned, was 41°.

18. Greater effects were obtained by washing and drying the platinized silver before it was immersed. In this way the needle, adjusted at 62°, would spring as far as 66o, indicating a transitory current of about 64o. Having now removed the glass weight, the needle took up a permanent position at 291, as at the beginning of the experiments.

19. When, instead of the platinized silver, the positive element (iron) was exposed to the air, whether simply or in conjunction with washing and drying, no appreciable increase of intensity was occasioned by its immersion. And although, on a repetition of the experiment, I sometimes observed slight effects, I conceive that they were owing to the power which the negative element seems to possess of collecting upou its surface the air held in solution by the circumambient liquid.

20. With an arrangement of platinized silver and amalgamated zinc, I obtained results of a similar though less striking character. The galvanometer indicated a permanent deflection of 629, and after washing and drying the platinized silver, I had a transitory deviation of 72o. The iminersion of the amalgamated zinc, after washiing and drying, produced no effect.

21. The maximum effects of immersion were produced in the following manner. A plate of silver was rubbed with a little nitric acid, and then exposed to a red heat, by means of which the film of nitrate of oxide of silver was decomposed and metallic silver reduced*. When the plate prepared in this way was associated in

By this process all the oxygen is not driven off, but a considerable quan tity remains adhering to the silver so tenaciously that it is not entirely removed by making the plate quite bright with glass paper. The oxygen thus deposited (it can hardly, I think, be considered as chemically combined with the silver) is the cause of the great intensity of the current immediately after immersion. By simply heating the silver to rodness the same general effects can be produced, though not to the same extent.

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