pairs of poles, namely, the metal B and the air above, and the metal A and the acid below, or a voltaic series composed of one metal and two fluids.

But as the air is a non-conductor, no current can yet be obtained. It is essential therefore to insert a conductor as its representative which shall retain the same relative condition of polarity, this polar condition being secured by its having a less affinity for oxygen than the zinc or primary metal.

N

Fig. 3.

A secondary plate of platinum, as in fig. 3, being substituted for the acid and the air of fig. 2, gives an arrangement of two equally polarized plates with their alternate poles in opposition; and having their lower poles joined by a conducting medium, they require only to be connected by their upper poles or electrodes to complete the circuit.

P

N

While separate, the chemical action is confined to the primary plate, and takes place in an upward direction; but immediately the electrodes are put into communication with each other, the action is diverted to the negative opposed to it in the conducting acid, and is now spread uniformly over the whole surface of the immersed metal. The polarization of the electrodes is thus shown to constitute an integral part of the battery itself; and these, by the addition of conducting-wires, are only made to undergo an extension of surface without alteration of electrical condition.

It is now obvious that placing between the electrodes any conducting substance capable of being decomposed must effect a corresponding action to that which takes place in the exciting fluid, and that an equal amount of chemical action will be effected at either end of the metals. Metallic contact, however, will reduce the two pairs of poles to one, as in the case of the horseshoe magnet, and thus effect a concentrated action.

In the first instance the secondary platinum plate only represents the polarity of the acid and the atmosphere; but on immersing the primary plate, and on this becoming equally polarized and combining with the oxygen of the electrolyte, there is a definite amount of hydrogen liberated, which retains its combining force unbalanced, and which then augments the charge of the secondary plate in an equal degree, and thus imparts to it a feeble degree of tension additional to the first power of the combination.

The chemical action occurring with the single metal chiefly at the surface of the fluid and but feebly within the acid lower

down, exerts only a trifling amount of force upon the secondary metal; but the instant the connexion is made through the electrodes, the whole of the electrolyte enclosed between the metal poles becomes electrolyzed and its ions separated, increasing the electromotive force in like proportion.

The contact of two dissimilar metals in air does not represent the two dissimilar metals of the battery, but simply corresponds with the two electric states of the primary metal alone. Scarcely any two metals have an equal affinity for oxygen, and any two of these placed together at once become polar and determine the mixed gases of the atmosphere to their respective poles. The combination which then takes place between the more oxidizable metal and the oxygen evolves or induces a certain amount of electrical force by which the combined metals and the adjacent portions of air become charged respectively positive and negative.

In the chemical action which takes place with the polarized primary alone, it was stated that the greatest amount of chemical action was found to occur near to the surfaces of air and acid in contact. The determination of oxygen from the atmosphere to the positive metal, combined with the electrolysis of the electrolyte, was here exhibited in the greater extent of oxidation and solution of the metal, and the less degree exhibited in the metal which had been partly excluded from the atmosphere.

That no current can be obtained from the contact of two metals in air is due to the fact that the atmosphere is not an electrolyte. It was distinctly defined by Faraday that no current is obtainable from chemical action unless by the decomposition of an electrolyte, the cation from which being absolutely indispensable for creating the tension of the secondary metal. The oxygen of the air having no cation to part with, is therefore unprovided with the means of accomplishing it.

The fact of this non-combination of the elements of the atmosphere constitutes the means of initiating the action of the battery. The electrolyte of the battery being held together by a combining force, cannot of its own accord separate itself into its component elements, but requires the introduction of some antagonistic force equivalent to or counterbalancing its cohesion, so as to set its elements free-to repolarize them in fact; this is accomplished by the introduction of the polarized metal, which, rendering the force equal on all sides, electrolyzes the water and allows its elements to rearrange themselves according to the polar influences then presented to them.

Were the atmosphere an electrolyte, it would then require some antecedent to effect its electrolysis, as the action must begin by a non-combination of elements, or a condition requiring no antecedent.

Norwich, September 1869.

XLV. Proceedings of Learned Societies.

ROYAL SOCIETY.

[Continued from p. 320.]

May 27, 1869.-Lieut.-General Sabine, President, in the Chair. THE following communications were read :

"Researches on Turacine, an Animal Pigment containing Copper." By A. W. Church, M.A. Oxon., Professor of Chemistry in the Royal Agricultural College, Cirencester.

From four species of Touraco, or Plantain-eater, the author has extracted a remarkable red pigment. It occurs in about fifteen of the primary and secondary pinion-feathers of the birds in question, and may be extracted by a dilute alkaline solution, and reprecipitated without change by an acid. It is distinguished from all other natural pigments yet isolated, by the presence of 5.9 per cent. of copper, which cannot be removed without the destruction of the colouring-matter itself. The author proposes the name turacine for this pigment. The spectrum of turacine shows two black absorptionbands, similar to those of scarlet cruorine; turacine, however, differs from cruorine in many particulars. It exhibits great constancy of composition, even when derived from different genera and species of Plantain-eater-as, for example, the Musophaga violacea, the Corythaix albo-cristata, and the C. porphyreolopha.

"On a New Arrangement of Binocular Spectrum-Microscope.' By William Crookes, F.R.S. &c.

The spectrum-microscope, as usually made, possesses several disadvantages: it is only adapted for one eye*; the prisms having to be introduced over the eyepiece renders it necessary to remove the eye from the instrument, and alter the adjustment, before passing from the ordinary view of an object to that of its spectrum and vice versa; the field of view is limited, and the dispersion comparatively small.

I have devised, and for some time past have been working with, an instrument in which the above objections are obviated, although at the same time certain minor advantages possessed by the ordinary instrument, such as convenience of examining the light reflected from an object, and comparing its spectrum with a standard spectrum, are not so readily associated with the present form of arrangement.

The new spectrum-apparatus consists of two parts, which are readily attached to an ordinary single or binocular microscope; and when attached they can be thrown in or out of adjustment by a touch of the finger, and may readily be used in conjunction with the polariscope or dichrooscope; object-glasses of high or low power can be used, although the appearances are more striking with a power of

Mr. Sorby in several of his papers (Proc. Roy. Soc. 1867, xv. p. 433; 'How to Work with the Microscope,' by L. Beale, F.R.S., 4th edition, p. 219) refers to a binocular spectrum-microscope; but he gives no description of it, and in one part says that it is not suited for the examination of any substance less than to of an inch in diameter.

4-inch focus or longer; and an object as small as a single corpuscle of blood can be examined and its spectrum observed.

The two additions to the microscope consist of the substage with slit &c., and the prisms in their box. The substage is of the ordinary construction, with screw adjustment for centring, and rack work for bringing it nearer to or withdrawing it from the stage. Its general appearance is shown in fig. 1, which represents it in position. A B is a plate of brass, sliding in grooves attached to the lower part

of the substage; it carries an adjustable slit, C, a circular aperture, D, 0.6 inch in diameter, and an aperture, O, inch square. A spring top enables either the slit or one of the apertures to be brought into the centre of the field without moving the eye from the eyepiece. Screw adjustments enable the slit to be widened or narrowed at will, and also varied in length. At the upper part of the substage is a

screw of the standard size, into which an object-glass of high power is fitted. E represents one in position. I generally prefer a 4-inch power; but it may sometimes be found advisable to use other powers here. The slit C and the object glass E are about 2 inches apart; and if light is reflected by means of the mirror along the axis of the instrument, it is evident that the object-glass E will form a small image of the slit C, about 0.3 inch in front of it. The milled head F moves the whole substage up or down the axis of the microscope, whilst the screws G and H, at right angles to each other, will bring the image of the slit into any desired part of the field. If the slide A B is pushed in so as to bring the circular aperture D in the centre, the substage arrangement then becomes similar to the old form of achromatic condenser. Beneath the slit C is an arrangement for holding an object, in case its surface is too irregular, or substance too dense, to enable its spectrum to be properly viewed in the ordinary way*.

Supposing an object is on the upper stage of the microscope (shown in fig. 2) and viewed by light transmitted from the mirror through the large aperture D and the condenser E, by pushing in the slide A B so as to bring the slit C into the field, and then turning the milled head F, it is evident that a luminous image of the slit C can be projected on to the object; and by proper adjustment of the focus, the object and the slit can be seen together equally sharp. Also, since the whole of the light which illuminated the object has been cut off, except that portion which passes through the slit, all that is now visible in the instrument is a narrow luminous line, in which is to be seen just so much of the object as falls within the space this line covers. By altering the slit-adjustments the length or width of the luminous line can be varied, whilst, by means of the rackwork attached to the upper stage, any part of the object may be superposed on the luminous line. The stage is supplied with a concentric movement, which permits the object to be rotated whilst in the field of view, so as to allow the image of the slit to fall on it in any direction. During this examination a touch with the finger will at any time bring the square aperture O, or the circular aperture D into the field, instead of the slit, so as to enable the observer to see the whole of the object; and in the same manner the slit can as easily be again brought into the field.

The other essential part of this spectrum-microscope consists of the prisms. These are enclosed in a box, shown at K (fig. 2). The prisms are of the direct-vision kind, consisting of three flint and two crown, and are altogether 16 inch long. The box screws into the end of the microscope-body at the place usually occupied by the object-glass; and the object-glass is attached by a screw in front of the prism-box. It is shown in its place at L. The prism-box is suffi

*In carrying out the experiments which were necessary before this spectrummicroscope could be made in its present complete form, I have been greatly assisted by Mr.C. Collins, Philosophical-Instrument Maker, 77 Great Tichfield Street, to whom I am also indebted for useful suggestions as to the most convenient arrangement of the different parts, so as to render them easily adapted to microscopes of ordinary construction.