by tapping, set their longest dimensions along the lines of force. Now, in this bar magnet the lines of force run along the magnet itself, and, were its particles capable of free motion, they also would set their longest dimensions parallel to the lines of force -that is to say, parallel to the length of the magnet. This, then, is the explanation given by M. De la Rive of the lengthening of the bar. The bar is composed of irregular crystalline granules; and, when magnetised, these granules tend to set their longest dimensions parallel to the axis of the bar. They succeed, partially, and produce a microscopic lengthening of the bar, which, suitably magnified, has been rendered visible to you. The explanation seems to me as satisfactory as it is acute.

Let me now endeavour to render these beautiful magnetic curves visible to you all. From an electric lamp turned on its back, a vertical cylinder of light issues. Over the aperture of the lamp are placed two small bar magnets, enclosed between two plates of glass. The vertical beam is received upon a looking-glass which reflects it on to the screen. In the path of this reflected beam is placed a lens, which projects upon the screen a magnified image of the two small magnets. And now I sprinkle fine iron sand on the plate of glass, and you see how it arranges itself under the operation of the magnets. A most beautiful display of the magnetic curves is now before you. And you observe, when I tap the glass, how the particles attach themselves by their ends, and how the curves close in upon each other. In the solid iron bar they also try to attach themselves thus, and close thus up; the consequence is that the longitudinal expansion is exactly counterbalanced by the transverse contraction, so that the volume of the bar remains unchanged.

But can we not bring a body with movable particles within an electro-magnetic coil? We can; and I will now, in conclusion, show you an experiment devised by Mr. Grove, which bears directly upon this question, but the sight of which, I believe, has hitherto been confined to Mr. Grove himself. At all events, I am not aware of its ever having been made before a large audience. This cylinder with glass ends contains a muddy liquid; the muddiness being produced by the magnetic oxide of iron which is suspended mechanically in water. Round

the glass cylinder are coiled five or six layers of covered copper wire; and here is a battery from which a current can be sent through the coil. First of all, I place the glass cylinder in the path of the beam from our electric lamp, and, by means of a lens, cast a magnified image of the end of the cylinder on the screen. That image at present possesses but feeble illumination. The light is almost extinguished by the suspended particles of magnetic oxide. But, if what has been stated regarding the lines of force through the bar of magnetised iron be correct, the particles of the oxide will suddenly set their longest dimensions parallel to the axis of the cylinder, and also in part set themselves end to end when the current is sent round them. More light will be thus enabled to pass; and now you observe the effect. The moment the circuit is established the disc upon the screen becomes luminous. When the current is interrupted, gloom supervenes; I re-establish it, and we have a luminous disc once more.

The apparatus before you was, as stated, really invented to examine whether any mechanical effect of this kind could be detected in diamagnetic bodies; but hitherto without result. And this leads me to remark on the large ratio which the failures of an original inquirer bear to his successes. The public see the success-the failure is known to the inquirer alone. The encouragement of his fellow-men, it is true, often cheers the investigator and strengthens his heart; but his main trials occur when there is no one near to cheer him, and when, if he works aright, he must work for duty and not for reputation. And this is the spirit in which work has been executed in this Institution, by a man who has, throughout his life, turned a deaf ear to such allurements as this age places within the reach of scientific renown; and it behoves every friend of this Institution to join in the wish that that man's spirit may continue to live within its walls, and that those who come after him may not shrink from his self-denial should they ever hope to merit a portion of his fame.

Biot found it impossible to work at his experiments on sound during the day in Paris; he was obliged to wait for the stillness of night. I found it almost equally difficult to make accurate experiments, requiring the telescope and scale, with the

instrument just described in London. Take a single experiment in illustration. The mirror was fixed so as to cause the crosshair of the telescope to cut the number 727 on the scale; a cab passed while I was observing-the mirror quivered, obliterating the distinctness of the figure, and the scale slid apparently through the field of view and became stationary at 694. I went upstairs for a book; a cab passed, and on my return I found the cross-hair at 686. A heavy waggon then passed, and shook the scale down to 420. Several car

riages passed subsequently; the figure on the scale was afterwards 350. In fact, so sensitive is the instrument that long before the sound of a cab is heard its approach is heralded by the quivering of the figures on the scale.

Various alterations which were suggested by the experiments were carried out by Mr. Becker, and the longer I worked with it the more mastery I obtained over it; but I did not work with it sufficiently long to perfect its arrangement. Some of the results, however, may be stated here.

At the beginning of a series of experiments the scale was properly fixed, and the pressure of the pointed vertical rod F, fig. 1, on the end of the iron bar, I, so regulated as to give the mirror a convenient position; then, before the bar was magnetised, the figure cut by the cross-hair of the telescope was read off. The circuit was then established, and a new number, depending on the altered length of the bar by its magnetisation, started into view. Then the circuit was interrupted, and the return of the mirror towards its primitive position was observed. The mirror, as stated, was drawn back to its first position by the spiral hair-spring shown in fig. 1. Here are some of the results :

Here the magnetisation of the bar produced an elongation expressed by 107 divisions of the scale, while the interruption of the circuit produced only a shrinking of 47 divisions. There was a tendency on the part of the bar, or of the mirror, to persist in the condition superinduced by the magnetism. The passing of a cab in this instance caused the scale to move from 517 to 534-that is, it made the shrinking 64 instead of 47. Tapping the bar produced the same effect.

The bar employed here was a wrought iron square core, 1-2 inch a side and 2 feet long.

The following tables will sufficiently illustrate the performance of the instrument in its present condition. In each case are given the figures observed before closing, after closing, and after interrupting the circuit. Attached to each table, also, are the lengthening produced by magnetising and the shortening consequent on the interruption of the circuit:

These constitute but a small fraction of the number of experiments actually made. There are very decided indications that the amount of elongation depends on the molecular condition of the bar. For example, a bar taken from a mass used in the manufacture of a great gun at the Mersey Iron-works suffered changes on magnetisation and demagnetisation considerably less than those recorded here. I hope to return to the subject.*

I owe these bars to the liberality of the proprietors of the Mersey Iron-works, through the friendly intervention of Mr. Mallet.

X. ON THE INFLUENCE OF MATERIAL AGGREGATION UPON THE MANIFESTATIONS OF FORCE*

[The following four essays are, for the most part, as stated in the note at the commencement, lecture-room summaries of the memoirs already presented to the reader. They, however, contain additional remarks and experiments which may be useful to the scientific teacher. To one of them are appended drawings, hitherto unpublished, of the moulds employed for compressing bismuth in the diamagnetic experiments.

The first of the series is a report of the first lecture delivered by me in the Royal Institution. It was given on Friday evening, February 11, 1853.]

THE system of the universe embraces two things,—an object acted upon, and an agent by which it is acted upon;-the object we call matter, and the agent we call force. Matter, in certain aspects, may be regarded as the vehicle of force; thus the luminiferous ether is the vehicle or medium by which the pulsations of the sun are transmitted to our organs of vision. Or to take a plainer case; if we set a number of billiard balls in a row and impart a shock to one end of the series, in the direction of its length, we know what takes place; the last ball will fly away, the intervening balls having served for the transmission of the shock from one end of the series to the other. Or we might refer to the conduction of heat. If, for example, it be required to transmit heat from the fire to a point at some distance from the fire, this may be effected by means of a conducting body-by the poker for instance: thrusting one end of the poker into the fire it becomes heated, the heat makes its way through the mass, and finally manifests itself at the other end. Let us endeavour to get a distinct idea of what we here call heat; let us first picture it to ourselves as an agent apart from the mass of the conductor, making its way among the particles of the latter, jumping from atom to atom,

* Proceedings of the Royal Institution, vol. i. p. 254.