65. This experiment was, as the others, conducted in the manner described at length under Exp. 7. Water chiefly* was decomposed;

1 3.5

and I ascertained, experimentally, that about of the intensity of the battery was expended in overcoming resistance to electrolysis. 3.52 +1.68 =2.03, the resistance to conduc

Thus I had 3:52

3.5

tion; and hence

(1.08)2
(1.88)2

retical heat.

30'

x 2.03 x 7.56 x =29.53, the theo60'

66. Exp. 13.-Two plates of copper, each of which was two inches broad, were secured at the distance of one inch apart, and immersed in two pounds of a saturated solution of sulphate of copper. Through this apparatus, a battery of ten zinc-iron pairs passed the mean current 1 Q, during half an hour. The heat thus produced, when properly corrected, was 5.8.

1

(1.88)°

30' × 5·5 +7.56 + 60'

67. In this case there was no resistance of electrolysis, and the action may be regarded simply as a transfer of copper from the positive to the negative electrode. All the obstruction, therefore, that was presented to the current, was resistance to conduction. Its mean was 5.5, whence we have 50-88, the theoretical heat. 68. We have thus arrived at the general conclusion, that the heat which is evolved by the proper action of any voltaic current is proportional to the square of the intensity of that current, multiplied by the resistance to conduction which it experiences. From this law the following conclusions are directly deduced:

69. 1st. That if the electrodes of a galvanic pair of given intensity be connected by any simply conducting body, the total voltaic heat generated by the entire circuit (provided always that no local action occurs in the pair) will, whatever may be the resistance to conduction, be proportional to the number of atoms (whether of water or of zinc) concerned in generating the current. For if the resistance to conduction be diminished, the quantity of current will be increased in the same ratio, and hence, according to the law (68), the quantity of heat which would thus be generated in a given time will be also proportionally increased; whilst of course the number of atoms which would be electrolyzed in the pair will be increased in the same proportion.

70. 2nd. That the total voltaic heat which is produced by any pair, is directly proportional to its intensity, and the number of atoms which are electrolyzed in it. For the quantity of current is proportional to the intensity of the pair, and consequently the quantity of heat evolved in a given time is proportional to the square of the

See Faraday on the electrolysis of nitric acid, "Experimental Researches" (752).

intensity of the pair, but the number of atoms electrolyzed is proportional, in the same time, to the simple ratio only of the current, or of the intensity of the pair.

71. And 3rd. That when any voltaic arrangement, whether simple or compound, passes a current of electricity through any substance, whether an electrolyte or not, the total voltaic heat which is generated in any time, is proportional to the number of atoms which are electrolyzed in each cell of the circuit, multiplied by the virtual* intensity of the battery.

72. Berzelius thinks that the light and heat produced by combustion are occasioned by the discharge of electricity between the combustible and the oxygen with which it is in the act of combination; and I am of opinion that the heat arising from this, and some other chemical processes, is the consequence of resistance to electric conduction. My experiments on the heat produced by the combustion of zinc turnings in oxygen (which, when sufficiently complete, I shall make public) strongly confirm this view; and the quantity of heat which Crawford produced by exploding a mixture of hydrogen and oxygen may be considered almost decisive of the question. In his unexceptionable experiments, one grain of hydrogen produced heat sufficient to raise one pound of water 9° 6. Now we know from Exp. 5, that the heat generated in one of Mr. Grove's 4.77 × 32.3

pairs by the electrolysis of

6

257 grains of zinc, is theoretically 3°46; and the heat which must in the same time have been generated by the metallic part of the circuit, which presented

0.06 0.427

the resistance 0.06, is ×3°460°·48; the total voltaic heat was therefore 3°.94. Hence the total heat which would have been evolved by the electrolysis of an equivalent, or 32-3 grains of 32.3 zinc, is × 3°.94 = 4°·95; which, when reduced to the capa25.7 city of one pound of water, is 99.9. But from the table of the intensities of voltaic arrangements (74), the intensity of Mr. Grove's pair, compared with the affinity of hydrogen for oxygen, is 0.93 whence, from (70), we have 9o.9 × 0·93 = 99.2, the heat which should be generated by the combustion of one grain of hydrogen, according to the doctrine of resistances; the result of Crawford is only 00.4.

1

73. I am aware that there are some anomalous conditions of the current which seem to militate against the general law (68), particularly when in the hands of Peltier it actually produces cold.†

If a decomposing cell be in the circuit, the virtual intensity of the battery is reduced in proportion to its resistance to electrolyzation.

+ If antimony and bismuth be soldered together, cold will be produced at the point of junction by the passage of the current from the bismuth to the antimony. Peltier, Annales de Chimie, vol. lvi, p. 371. In his paper, however, a misprint has inverted the direction of the current.

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.

of copper

Mr. Smee's

Mr. Sturgeon's Dilute sulphuric acid

Iron. Amalgamated zinc

Platinized silver. Amalg. zinc
Dilute sulphuric acid

1

3.33

1

3:58

Copper. Algamated zinc

Dilute sulphuric acid

1

Constant Intensities.

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.

it

Broom Hill, Pendlebury near Manchester,

March 25th, 1841.

P.S. 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 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) observed that iron is sometimes more efficient than platinum in voltaic association with amalgamated zinc.

August 11, 1841.

J. P. J.

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 conduction.

2. We have seen that when 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 known 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, platinized 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. The needle indicated a pretty constant deviation of 20°, 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 true 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 platinized silver and a stout iron wire had been passed. On connecting the metals with the galvanometer (5) its needle was deflected to 3210, and on shaking the flask very briskly it could not be made to advance further than 34. 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 50°; in five minutes more to 49°, and in another five minutes to 481. 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 "cations."