and on this place a magnetic needle within an eighth of an inch of the wire. The apparatus, set standing in a proper vessel, is to be so turned that the copper horizontal wire shall coincide with the direction of the magnetic needle. Pour a sufficiency of very dilute sulphuric acid into the vessel, and instantly the needle will be deflected between 30° and 40°. This is the same degree of deflection which the galvanometer needle suffered in Colladon's experiment with common electricity; but in order to obtain that amount, he was obliged to use a galvanometer coil consisting of 500 turns; that is, he was under the necessity of employing the combined effect of 500 wires carrying an intense power of common electricity; whereas in the experiment just described, where the effect was truly voltaic, the same amount of deflection was produced by a single wire carrying the most feeble voltaic electricity.

The experiments of M. Colladon were repeated and varied by Professor Faraday. He employed a Leyden battery having a surface of 3510 square inches of coated glass. By successive discharges of this battery through the galvanometer, conducted by a wet thread 4 feet long, the needle at length suffered deflection to the amount of 40° on each side of the line of rest. He obtained the same deflection also by electricity direct from the prime conductor, without the battery*.

The plate of the machine used by Faraday, as described by him, is 50 inches in diameter; it is furnished with two sets of rubbers; one revolution of the plate will give from ten to twelve sparks from the conductor, each 1 inch long. The battery which produced deflection on the needle had been charged with forty revolutions, consisting of 440 one-inch sparks; and as it was found necessary to repeat the discharge several times in order to produce a deflection of 40°, at least 2000 one-inch sparks were required for that purpose.

These deflections, however, both in the case of the battery and of the conductor, derived assistance from other sources beside electricity, which greatly magnified their amount. The swings of the needle were promoted, from very small arcs, to one of 40° by alternate circulation and interruption of the electric current. Hence the amount of angular deviation was rather a semblance than the reality of voltaic deflection. A heavy pendulum may be made to oscillate in considerable arcs by causing the weakest force to act on it at intervals corresponding with the time of its oscillations. Since the deflection, assisted as it was by the method of production, was but 40°, how feeble must have been the force that produced it!

Mr. Armstrong, with an hydro-electric machine which dis*Researches, p. 85.

charged torrents of electricity and gave sparks 22 inches long, could only produce a deflection of 20° or 30°.

11

In my experiments already described, a bit of copper wire weighing and of a grain, in connexion with a platinum wire, by a momentary contact with nitric acid, caused the needle to whirl round three times, that is, with an effect incomparably greater than the maximum in the experiments of Colladon and Faraday. An atom of gold-leaf weighing dth of a grain caused deflection to 180°. In neither case was perhaps the ten-thousandth part of a grain of metal dissolved before deflection commenced. Is it possible that such a chemical action, which almost exceeds comprehension for minuteness of effect and of duration, should, in its results on the galvanometer, rival the enormous powers of the plate machine and the hydro-electric machine, if the agent in both cases had been the same? Some scientific questions are best decided by common sense; and if common sense decides that a particle of copper scarcely visible dipped in nitric acid for an instant, producing no obvious effect of electricity, does notwithstanding evolve more of that fluid than a hydro-electric machine, which pours out an incessant stream of long sparks, I must then admit that my arguments are worthless.

But there is one experiment of Faraday which deserves particular notice. He found that, without any Leyden battery, he could produce deflection merely by conducting electricity from the prime conductor to one end of the galvanometer coil, while the other end was in communication with a discharging train, that is, a metallic connexion with the gas-pipes and water-pipes in the street. Thus the electric fluid passed from the conductor directly through the galvanometer, and hence to the common reservoir. The principal feature in this experiment is, that no negative conductor was employed, nor were means used for bringing the negative state of electricity into operation; the deflection was therefore obtained by a current of positive electricity only. The condition for producing deflection by voltaic electricity, is invariably by means of two polar conductors, one of which is said to carry positive electricity, the other negative, no matter whether these be different states or different fluids; both of the poles must be in operation, and the moment either is withdrawn, by interrupting the circuit, the power that causes deflections, and all the other phænomena, ceases to act. But in Professor Faraday's experiment this condition was not fulfilled. There was no connexion of two polar conductors with the coil; positive and negative electricity were not in operation; there was no circuit; in fine, the circumstances were totally different from those under which voltaic deflection is produced. In the voltaic series, the negative wire is not passive, like the discharging

train; it is as active as the positive wire; it collects round it a whole class of bodies with as much energy as the positive wire does an opposite class; how then are we to understand its absence in the deflections obtained by Faraday with common electricity in the experiment described? If we look upon the influence acting in two indispensable poles as the cause of voltaic phænomena, can we consider phænomena produced by one pole as emanating from the same cause?

For my own part, I cannot dismiss from my mind a strong impression that the agent in Faraday's experiment was not the same as that which causes voltaic phænomena. Nay, more than this, if it be proved by his experiment that common electricity does not require a twofold polar arrangement in order to produce deflections, I cannot see what the use is of the two poles used in his and Colladon's experiments with the Leyden battery; one of them must have been superfluous. If this be so, we arrive at this general proposition, that voltaic electricity is composed of elements existing in such ratio, and so combined and modified, that it must be brought to bear upon the subject of its action by means of two poles simultaneously and equally energetic; while the proportions and mode of combination in the common electric fluid are such that it produces the same effect with one pole only. Thus, by Faraday's experiments, if my reasonings be correct, an important difference is established, instead of an identity; other facts and arguments of the same tendency will be hereafter brought forward.

I now take leave of this part of the subject, and proceed to consider some other evidences which have been brought forward in support of the affirmed efficiency of quantity to explain the differences observable between the effects of common and voltaic electricity. It is a subject deserving full consideration, as on this foundation is raised the whole superstructure.

[To be continued.]

XXXII. On Rubian and its Products of Decomposition.
By EDWARD SCHUNCK, F.R.S.*

PART 1.

AMONG the many discussions to which the subject of madder

has given rise among chemists, there is none which is calculated to excite so much interest as that concerning the state in which the colouring matter originally exists in this root, and there is no part of this extensive subject which is at the same

* From the Philosophical Transactions for 1851, part ii.; having been received by the Royal Society January 9, and read February 13, 1851.

time involved in such obscurity. It is a well-known fact that the madder root is not well adapted for the purposes of dyeing until it has attained a growth of from eighteen months to three years, and that after being gathered and dried it gradually improves for several years, after which it again deteriorates. During the time when left to itself, especially if in a state of powder, it increases in weight and bulk, in consequence probably of absorption of moisture from the air, and some chemical change is effected, which, though not attended by any striking phænomena, is sufficiently well indicated by its results. There are few chemical investigations that have thrown any light on the nature of the process which takes place during this lapse of time, and in fact most of the attempts to do so have merely consisted of arguments based on analogy. It has been surmised that the process is one of oxidation, and that the excess of atmospheric air is consequently necessary. We are indeed acquainted with cases, in which substances of well-defined character and perfectly colourless, as for instance orcine and hematoxyline, are converted by the action of oxygen, or oxygen and alkalies combined, into true colouring matters. A more general supposition is, that the process is one of fermentation, attended perhaps by oxidation, and in confirmation of this view the formation of indigo-blue from a colourless plant, by a process which has all the characters of one of fermentation, may be adduced. What the substance is however on which this process of oxidation or fermentation takes effect, what the products are which are formed by it, whether indeed the change is completed as soon as the madder has reached the point when it is best adapted for dyeing, or whether further changes take place when it is mixed with water and the temperature raised during the process of dyeing, are questions which have never been satisfactorily answered, if answered at all. It has indeed been suspected by several chemists, that there exists originally some substance in madder, which by the action of fermentation or oxidation is decomposed and gives rise by its decomposition to the various substances endowed either with a red or yellow colour, which have been discovered during the chemical investigations of this root. That several of these substances are merely mixtures, and some of them in the main identical, has been satisfactorily proved by late investigators. But there still remain a number, which, though extremely similar, have properties sufficiently marked to entitle them to be considered as distinct.

In my papers on the colouring matters of madder*, I have described four substances derived from madder, only one of which is a true colouring matter, but all of them capable, under *Phil. Mag. August 1848, and September 1849.

certain circumstances, as for instance in combination with alkalies, of developing red or purple colours of various intensity. To seek for a common origin for these various bodies so similar to one another and yet distinct, is very natural, and the discovery of it no improbable achievement.

Persoz asserts the probability of this view in the following words:"We may hence venture to conclude that the colouring matters which we extract from fabrics dyed with madder, as well as the alizarine which is obtained by submitting the products derived from madder to sublimation, do not exist readyformed in this root, and are only products derived from another substance which has not yet been isolated..... From numerous experiments which I have made on this subject, it follows that the colouring matter of madder may be compared, in respect to the derivatives to which it gives rise, to tannin, so that I do not despair of being able, as far as regards their metamorphoses, to establish a parallel between the products derived from madder and those obtained from tannin. If it should be possible to confer on the former that tendency to assume regular forms with which the latter are endowed, the separation of the proximate colouring or colour-giving (colorable) matters of madder will be easy, and it will thus be possible to establish their elementary composition and thence their relation to one another."

To Mr.J. Higgin is due the merit of having first called attention to the fact, that important changes take place during the process of dyeing with madder, which can only be explained by supposing that an actual formation of colouring matter takes place during the process. In his In his paper On the Colouring Matters of Maddert, Mr. Higgin has detailed his experiments on that peculiar substance discovered in madder by Kuhlmann and called by him Xanthine. I have shown, on a former occasion, that the xanthine of Kuhlmann and other investigators is not a pure substance, but a mixture of two distinct substances. This fact however does not affect the correctness of Mr. Higgin's conclusions, the general accuracy of which I shall have great pleasure in confirming in the course of this paper. The presence of xanthine is easily ascertained by the deep yellow colour and intensely bitter taste which it communicates to cold water. Guided by these two tests, Mr. Higgin arrived at the conclusion, that in an infusion of madder, made with cold or tepid water, when left to itself, or more rapidly when heated to 120 or 130° Fahr., the xanthine gradually disappears and there is formed a gelatinous or flocculent substance, which possesses all the tinctorial power originally belonging to the infusion, while the liquid has lost all trace of any such power, and that as alizarine is the only *Traité de l'Impression des Tissus, t. i. p. 501. + Philosophical Magazine for Oct. 1848.