copper cell, are in the same mercury hole. Fig. 6. represents

c c c c c c c c c c c c c c c c c c c с

the upper surface of the trough by which all the zinc and copper plates are made to act as 20 circles.

From the preceding description of our battery, it is evident that the whole 20 zinc plates and copper cells may, by substituting one mercury trough for another, be made to act as a single pair, or as 2, 3, 4, 5, 6, 10, or 20 voltaic circles, and thus be made to supply the place of the calorimotor and of the battery hitherto used for electro-magnetic experiments.

So enormous is the quantity of electricity circulated by this battery when all the zinc and copper plates act as a single circle, that, on one occasion, after having acted without interruption for more than an hour, it rendered powerfully magnetic an electro-magnet on which were coiled 39 thick copper wires, each about 35 feet long, while the mercury in which the wires of the zinc plates were immersed, was connected by 6 copper wires, each of an inch thick and about 6 inches long, with the mercury in communication with the wires of the copper cells. On the fifth day it was tried: after having been in action without interruption for more than two hours, this battery melted very rapidly platina wire th of an inch thick, and deflagrated in a most brilliant manner copper and iron wire about th of an inch thick.

By this battery, with the aid of an electro-magnet, a current of electricity may be produced which will equal in intensity that of a battery containing 1000 voltaic circles. It is well known that when the connexion between the helix of an electro-magnet and the voltaic battery is broken, a current of electricity is, at the moment of breaking the connexion, made to flow through the helix; and that when the helix is long, that current is capable of giving a shock to any person who holds in each hand a copper cylinder in conducting communication with the ends of the helix. By experiments on the best means of obtaining the shock from the electro-magnet, I have found that the shock increases, within certain limits, with the length and thinness of the bar of soft iron, and with the length of the heliacal coil, as far perhaps as 200 feet, and in proportion, or nearly in proportion, to the number of plates in the voltaic battery from which the current of electricity is passed through the helix. The shock does not increase in proportion to the number

of plates unless they are large. The electro-magnet which I first used was a straight bar of soft iron, about 2 feet long, and an inch thick. On this bar were coiled two copper wires, each about 200 feet long. The voltaic battery consisted of 14 pairs of zinc and of as many double copper plates: each plate was about 7 inches square. The end of the first coil and the beginning of the second were immersed into the same cup of mercury, the voltaic current was passed through the first coil only, and the shock was taken by making a communication with the beginning of the first coil and with the end of the second. When the current of electricity was passed through the helix from one pair of plates, the shock received on breaking contact with the battery was equal to that of a battery containing 20 pairs of plates. When two pairs of plates were used, the shock appeared to be doubled; with three voltaic circles, it appeared to be trebled; and with every increase in the number of voltaic circles, there appeared to be a proportional increase of the shock. With the 14 pairs of plates the shock was so strong that a person who took it, from an electro-magnet on which there were four coils of wire, felt the effects of it for several days. With a battery of 4-inch plates the shock increased with the number of plates, but not so rapidly as when large plates were used. I am inclined to think that with a battery of 4-inch plates, the shock increases but little when the number of plates exceeds a hundred. I could not induce any one to take the shock from the electro-magnet when a greater number than 16 of our large plates were used. With 16 of them the shock was exceedingly strong, although the acid mixture employed in charging the battery was very weak; and, from experience, I know that the electro-magnetic effects of a battery depend very much on the strength of the charge.

From all the experiments which I have made on the magneto-electric shock, I think I may fairly conclude, that, if 2000 feet of wire were coiled on a bar of soft iron 6 feet long and an inch thick, a shock might be obtained with the aid of a single pair of plates, which would equal that of a battery of 100 voltaic circles. Hence, since the shock increases in proportion to, or, at least, very rapidly with the number of plates, when they are large, the shock given by such an electro-magnet magnetized by our battery of 20 pairs of plates, should nearly equal, or perhaps exceed that of a battery of 1000 voltaic circles. Hence, by our battery of 20 pairs of plates, an electric current of the highest intensity may be produced. This battery then supplies the place of all the various kinds of galvanie batteries.

The shock given by the electro-magnet may be obtained

as often as the connexion of the helix with the battery is broken. Now I have devised a small instrument by which communication with the battery may be broken and renewed 3000 or 4000 times in a minute. Thus 3000 or 4000 shocks may be received, and 3000 or 4000 electric currents of the highest intensity may, in the space of one minute, be passed through water, charcoal, metallic wires, or any other body. It should be remembered that the voltaic current from the battery should not be passed through more than 200 feet of the heliacal coil, and that the shock should be taken from the whole length of the helix.

When a voltaic current passes through a very long wire from a single pair of plates, the wire will give a shock at the moment of breaking contact with the battery. I have found that this as well as the shock from the electro-magnet increases with the number of plates.

I have made a great variety of experiments on electromagnets. My object in these experiments was to ascertain four things: first, on what the quantity of attraction depends; secondly, on what the distance at which that attraction is exerted depends; thirdly, on what the shock depends; and fourthly, whether by a voltaic current from a large battery, a permanent magnet could be made, which would induce on soft iron magnetism equal to that which is given to an electromagnet by a battery containing 20 large plates, or 300 fourinch plates. In these experiments I employed three different voltaic batteries, and electro-magnets of various forms. I used the large battery already described; a small battery of 14 pairs of plates, in which each zinc plate was seven inches square; and a Wollaston battery, containing 280 pairs of fourinch plates. Some of my electro-magnets were straight, and others of the horse-shoe form, and one was a square: the iron bars varied in length from 20 inches to six feet, and in thickness from two inches to half an inch. On one of these were coiled 39 copper wires, on another four, on a third three, and on others there was only one wire.

From the results of these experiments, I have deduced the following conclusions: First, that the quantity of attraction increases with the length of the bar of soft iron, at least as far as six feet, and with the thinness till it becomes about an inch; and that it increases nearly in proportion to the number of plates (when they are large) in the battery by which the electromagnet is magnetized. When the plates are only four inches. square the attraction increases, but slowly when the number exceeds 100. Secondly, that the distance at which attraction is exerted, increases also with the length and thickness of the

iron bar, and with the number of plates when they are large; but with small plates the increase is very gradual when their number exceeds a hundred. With twenty of our large plates, an iron bar, nearly 3 lbs. weight, was attracted to a horse-shoe electro-maguet through the distance of an inch, and with ten plates the same bar was attracted to the same magnet, only through the distance of about half an inch. Again, with the twenty plates, the attraction of the same magnet for a sewingneedle was sensible at the distance of 15 inches, and with ten plates the attraction was sensible at the distance of 10 inches. Thirdly, that the shock from the electro-magnet increases within certain limits with the length and thinness of the iron bar, and nearly in proportion to the number of plates when they are large.

When the voltaic current was sent from a battery of 280 four-inch plates, through the heliacal wire coiled round a steel bar about 20 inches long and an inch thick, the steel became almost as strongly magnetic as if it were iron; and when the connexion with the battery was broken, the steel did not retain more than about T of its magnetism.

In a paper published in the last (August) number of the Philosophical Magazine, Dr. Ritchie says that the use of the electro-magnet in the apparatus for continued rotation was long since abandoned, because it was incapable of inducing magnetism in an iron bar at a distance. Now he will find that, if instead of a single copper and zinc plate, a battery of 20 pairs of large plates, or of 200 small ones be used, the electromagnet will have a greater power of inducing magnetism at a distance than any permanent magnet.

The advantages of the battery I have described are, first, that it supplies the place of all the various kinds of voltaic batteries, of the battery for producing a large quantity of electricity of low intensity, of the battery for exciting a large quantity of electricity of the intensity necessary for the rapid fusion and deflagration of metallic wires, and of the battery for producing an electric current of high intensity; and secondly, that it enables a person to compare the power of the very same zinc and copper plates acting as a single pair, with their power when they act as 2, 3, 4, 5, 6, 10, or 20 voltaic circles. NICHOLAS CALLAN.

LXXXIX. Observations on certain Liquids obtained from Caoutchouc by Distillation. By JOHN DALTON, D.C.L., F.R.S., &c.*

Manchester, November 10, 1836.

DR. GREGORY having published in the last number of the Philosophical Magazine (p. 321.) some interesting experiments and observations on the liquid obtained by the distillation of caoutchouc, I have thought it would be acceptable to that gentleman, as well as to the public, to be made acquainted with the results I obtained from the same subject about two years and a half since, more especially as my experiments chiefly tend to establish additional properties to those deduced in Dr. Gregory's essay. For this purpose I send the Editors a copy of my paper read before the Literary and Philosophical Society of Manchester, on the 17th of October, 1834, which has not been published. I think it is obvious from what follows that most or all the varieties of vegetable combustible products of the oily character must be constituted of central atoms of carbon, oxygen, or carbonic oxide, along with a number of atoms of binolefiant gas, placed alternately around the central atom; and that the repeated distillation of them, at first with a greater and then with a less heat, gradually attenuates the compound atom, till at last it becomes one or two atoms of binolefiant gas slightly adhering to the less volatile parts of the oil, so that the gas, when not under sufficient restraint, expands into the atmosphere at the ordinary temperature.

Observations, &c. Read October 17, 1834.

The article caoutchouc is too generally known to require a particular description; it may suffice to observe that it is obtained from the milky juice of certain trees in South America, which juice is procured by incisions made in the bark of the trees. When the watery part of the juice, which constitutes more than half its weight, is evaporated, there remains a solid elastic substance, which is the caoutchouc. The properties and peculiarities of this singular substance have been mostly described in books of chemistry and other works, and therefore need not here be enumerated. Some new characteristics, however, seem lately to have been discovered by subjecting the article to repeated distillation, and it is upon those that we are about to make a few observations.

Most if not all vegetable products are liable to be decomposed by heat. They are mostly resolved into solid, liquid, and elastic substances, according, in some degree, to the temperature. The instance of the destructive distillation of

* Communicated by the Author.