Part 14 (1/2)
On March 8, 1871, he accepted the chair of Experimental Physics in the University of Cambridge. This chair was founded in consequence of an offer made by the Duke of Devons.h.i.+re, the Chancellor of the University, to build and equip a physical laboratory for the use of the university. In this capacity Maxwell's first duty was to prepare plans for the laboratory. With this view, he inspected the laboratories of Sir William Thomson at Glasgow, and of Professor Clifton at Oxford, and endeavoured to embody the best points of both in the new building. The result was that, in conjunction with Mr. W.
M. Fawcett, the architect, he secured for the university a laboratory n.o.ble in its exterior, and admirably adapted to the purposes for which it is required. The ground-floor comprises a large battery-room, which is also used as a storeroom for chemicals; a workshop; a room for receiving goods, communicating by a lift with the apparatus-room; a room for experiments on heat; balance-rooms; a room for pendulum experiments, and other investigations requiring great stability; and a magnetic observatory. The last two rooms are furnished with stone supports for instruments, erected on foundations independent of those of the building, and preserved from contact with the floor. On the first floor is a handsome lecture-theatre, capable of accommodating nearly two hundred students. The lecture-table is carried on a wall, which pa.s.ses up through the floor without touching it, the joists being borne by separate brick piers. The lecture-theatre occupies the height of the first and second floors; its ceiling is of wood, the panels of which can be removed, thus affording access to the roof-princ.i.p.als, from which a load of half a ton or more may be safely suspended over the lecture-table. The panels of the ceiling, adjoining the wall which is behind the lecturer, can also be readily removed, and a ”window” in this wall communicates with the large electrical-room on the second floor. Access to the s.p.a.ce above the ceiling of the lecture-theatre is readily obtained from the tower.
Adjoining the lecture-room is the preparation-room, and communicating with the latter is the apparatus-room. This room is fitted with mahogany and plate-gla.s.s wall and central cases, and at present contains, besides the more valuable portions of the apparatus belonging to the laboratory, the marble bust of James Clerk Maxwell, and many of the home-made pieces of apparatus and other relics of his early work. The rest of the first floor is occupied by the professor's private room and the general students' laboratory.
Throughout the building the brick walls have been left bare for convenience in attaching slats or shelves for the support of instruments. The second floor contains a large room for electrical experiments, a dark room for photography, and a number of private rooms for original work. Water is laid on to every room, including a small room in the top of the tower, and all the windows are provided with broad stone ledges without and within the window, the two portions being in the same horizontal plane, for the support of heliostats or other instruments. The building is heated with hot water, but in the magnetic observatory the pipes are all of copper and the fittings of gun-metal. Open fireplaces for basket fires are also provided. Over the princ.i.p.al entrance of the laboratory is placed a stone statue of the present Duke of Devons.h.i.+re, together with the arms of the university and of the Cavendish family, and the Cavendish motto, ”Cavendo Tutus.” Maxwell presented to the laboratory, in 1874, all the apparatus in his possession. He usually gave a course of lectures on heat and the const.i.tution of bodies in the Michaelmas term; on electricity in the Lent term; and on electro-magnetism in the Easter term. The following extract from his inaugural lecture, delivered in October, 1871, is worthy of the attention of all students of science:--
Science appears to us with a very different aspect after we have found out that it is not in lecture-rooms only, and by means of the electric light projected on a screen, that we may witness physical phenomena, but that we may find ill.u.s.trations of the highest doctrines of science in games and gymnastics, in travelling by land and by water, in storms of the air and of the sea, and wherever there is matter in motion.
The habit of recognizing principles amid the endless variety of their action can never degrade our sense of the sublimity of nature, or mar our enjoyment of its beauty. On the contrary, it tends to rescue our scientific ideas from that vague condition in which we too often leave them, buried among the other products of a lazy credulity, and to raise them into their proper position among the doctrines in which our faith is so a.s.sured that we are ready at all times to act on them.
Experiments of ill.u.s.tration may be of very different kinds. Some may be adaptations of the commonest operations of ordinary life; others may be carefully arranged exhibitions of some phenomenon which occurs only under peculiar conditions. They all, however, agree in this, that their aim is to present some phenomenon to the senses of the student in such a way that he may a.s.sociate with it some appropriate scientific idea. When he has grasped this idea, the experiment which ill.u.s.trates it has served its purpose.
In an experiment of research, on the other hand, this is not the princ.i.p.al aim.... Experiments of this cla.s.s--those in which measurement of some kind is involved--are the proper work of a physical laboratory. In every experiment we have first to make our senses familiar with the phenomenon; but we must not stop here--we must find out which of its features are capable of measurement, and what measurements are required in order to make a complete specification of the phenomenon. We must then make these measurements, and deduce from them the result which we require to find.
This characteristic of modern experiments--that they consist princ.i.p.ally of measurements--is so prominent that the opinion seems to have got abroad that, in a few years, all the great physical constants will have been approximately estimated, and that the only occupation which will then be left to men of science will be to carry these measurements to another place of decimals.
If this is really the state of things to which we are approaching, our laboratory may, perhaps, become celebrated as a place of conscientious labour and consummate skill; but it will be out of place in the university, and ought rather to be cla.s.sed with the other great workshops of our country, where equal ability is directed to more useful ends.
But we have no right to think thus of the unsearchable riches of creation, or of the untried fertility of those fresh minds into which these riches will continually be poured.... The history of Science shows that, even during that phase of her progress in which she devotes herself to improving the accuracy of the numerical measurement of quant.i.ties with which she has long been familiar, she is preparing the materials for the subjugation of new regions, which would have remained unknown if she had been contented with the rough methods of her early pioneers.
Maxwell's ”Electricity and Magnetism” was published in 1873. Shortly afterwards there were placed in his hands, by the Duke of Devons.h.i.+re, the Cavendish Ma.n.u.scripts on Electricity, already alluded to. To these he devoted much of his spare time for several years, and many of Cavendish's experiments were repeated in the laboratory by Maxwell himself, or under his direction by his students. The introductory matter and notes embodied in ”The Electrical Researches of the Honourable Henry Cavendish, F.R.S.,” afford sufficient evidence of the amount of labour he expended over this work. The volume was published only a few weeks before his death. Another of Maxwell's publications, which, as a text-book, is unique and beyond praise, is the little book on ”Matter and Motion,” published by the S.P.C.K.
In 1878 Maxwell, at the request of the Vice-Chancellor, delivered the Rede Lecture in the Senate-House. His subject was the telephone, which was just then absorbing a considerable amount of public attention.
This was the last lecture which he ever gave to a large public audience.
It was during his tenure of the Cambridge chair that one of the cottages on the Glenlair estate was struck by lightning. The discharge pa.s.sed down the damp soot and blew out several stones from the base of the chimney, apparently making its way to some water in a ditch a few yards distant. The cottage was built on a granite rock, and this event set Maxwell thinking about the best way to protect, from lightning, buildings which are erected on granite or other non-conducting foundations. He decided that the proper course was to place a strip of metal upon the ground all round the building, to carry another strip along the ridge-stay, from which one or more pointed rods should project upwards, and to unite this strip with that upon the ground by copper strips pa.s.sing down each corner of the building, which is thus, as it were, enclosed in a metal cage.
After a brief illness, Maxwell pa.s.sed away on November 5, 1879. His intellect and memory remained perfect to the last, and his love of fun scarcely diminished. During his illness he would frequently repeat hymns, especially some of George Herbert's, and Richard Baxter's hymn beginning
”Lord, it belongs not to my care.”
”No man ever met his death more consciously or more calmly.”
It has been stated that Thomas Young propounded a theory of colour-vision which a.s.sumes that there exist three separate colour-sensations, corresponding to red, green, and violet, each having its own special organs, the excitement of which causes the perception of the corresponding colour, other colours being due to the excitement of two or more of these simple sensations in different proportions. Maxwell adopted blue instead of violet for the third sensation, and showed that if a particular red, green, and blue were selected and placed at the angular points of an equilateral triangle, the colours formed by mixing them being arranged as in Young's diagram, all the shades of the spectrum would be ranged along the sides of this triangle, the centre being neutral grey. For the mixing of coloured lights, he at first employed the colour-top, but, instead of painting circles with coloured sectors, the angles of which could not be changed, he used circular discs of coloured paper slit along one radius. Any number of such discs can be combined so that each shows a sector at the top, and the angle of each sector can be varied at will by sliding the corresponding disc between the others. Maxwell used discs of two different sizes, the small discs being placed above the larger on the same pivot, so that one set formed a central circle, and the other set a ring surrounding it. He found that, with discs of five different colours, of which one might be white and another black, it was always possible to combine them so that the inner circle and the outer ring exactly matched. From this he showed that there could be only three conditions to be satisfied in the eye, for two conditions were necessitated by the nature of the top, since the smaller sectors must exactly fill the circle and so must the larger.
Maxwell's experiments, therefore, confirmed, in general, Young's theory. They showed, however, that the relative delicacy of the several colour-sensations is different in different eyes, for the arrangement which produced an exact match in the case of one observer, had to be modified for another; but this difference of delicacy proved to be very conspicuous in colour-blind persons, for in most of the cases of colour-blindness examined by Maxwell the red sensation was completely absent, so that only two conditions were required by colour-blind eyes, and a match could therefore always be made in such cases with four discs only. Holmgren has since discovered cases of colour-blindness in which the violet sensation is absent. He agrees with Young in making the third sensation correspond to violet rather than blue. Maxwell explained the fact that persons colour-blind to the red divide colours into blues and yellows by the consideration that, although yellow is a complex sensation corresponding to a mixture of red and green, yet in nature yellow tints are so much brighter than greens that they excite the green sensation more than green objects themselves can do, and hence greens and yellows are called yellow by such colour-blind persons, though their perception of yellow is really the same as perception of green by normal eyes. Later on, by a combination of adjustable slits, prisms, and lenses arranged in a ”colour-box,” Maxwell succeeded in mixing, in any desired proportions, the light from any three portions of the spectrum, so that he could deal with pure spectral colours instead of the complex combinations of differently coloured lights afforded by coloured papers. From these experiments it appears that no ray of the solar spectrum can affect one colour-sensation alone, so that there are no colours in nature so pure as to correspond to the pure simple sensations, and the colours occupying the angular points of Maxwell's diagram affect all three colour-sensations, though they influence two of them to a much smaller extent than the third. A particular colour in the spectrum corresponds to light which, according to the undulatory theory, physically consists of waves all of the same period, but it may affect all three of the colour-sensations of a normal eye, though in different proportions. Thus, yellow light of a given wave-length affects the red and green sensations considerably and the blue (or violet) slightly, and the same effect may be produced by various mixtures of red or orange and green. For his researches on the perception of colour, the Royal Society awarded to Clerk Maxwell the Rumford Medal in 1860.
Another optical contrivance of Maxwell's was a wheel of life, in which the usual slits were replaced by concave lenses of such focal length that the picture on the opposite side of the cylinder appeared, when seen through a lens, at the centre, and thus remained apparently fixed in position while the cylinder revolved. The same result has since been secured by a different contrivance in the praxinoscope.
Another ingenious optical apparatus was a real-image stereoscope, in which two lenses were placed side by side at a distance apart equal to half the distance between the pictures on the stereoscopic slide.
These lenses were placed in front of the pictures at a distance equal to twice their focal length. The real images of the two pictures were then superposed in front of the lenses at the same distance from them as the pictures, and these combined images were looked at through a large convex lens.
The great difference in the sensibility to different colours of the eyes of dark and fair persons when the light fell upon the _fovea centralis_, led Maxwell to the discovery of the extreme want of sensibility of this portion of the retina to blue light. This he made manifest by looking through a bottle containing solution of chrome alum, when the central portion of the field of view appears of a light red colour for the first second or two.
A more important discovery was that of double refraction temporarily produced in viscous liquids. Maxwell found that a quant.i.ty of Canada balsam, if stirred, acquired double-refracting powers, which it retained for a short period, until the stress temporarily induced had disappeared.
But Maxwell's investigations in optics must be regarded as his play; his real work lay in the domains of electricity and of molecular physics.
In 1738 Daniel Bernouilli published an explanation of atmospheric pressure on the hypothesis that air consists of a number of minute particles moving in all directions, and impinging on any surface exposed to their action. In 1847 Herapath explained the diffusion of gases on the hypothesis that they consisted of perfectly hard molecules impinging on one another and on surfaces exposed to them, and pointed out the relation between their motion and the temperature and pressure of a gas. The present condition of the molecular theory of gases, and of molecular science generally, is due almost entirely to the work of Joule, Clausius, Boltzmann, and Maxwell. To Maxwell is due the general method of solving all problems connected with vast numbers of individuals--a method which he called the statistical method, and which consists, in the first place, in separating the individuals into groups, each fulfilling a particular condition, but paying no attention to the history of any individual, which may pa.s.s from one group to another in any way and as often as it pleases without attracting attention. Maxwell was the first to estimate the average distance through which a particle of gas pa.s.ses without coming into collision with another particle. He found that, in the case of hydrogen, at standard pressure and temperature, it is about 1/250000 of an inch; for air, about 1/389000 of an inch. These results he deduced from his experiments on viscosity, and he gave a complete explanation of the viscosity of gases, showing it to be due to the ”diffusion of momentum” accompanying the diffusion of material particles between the pa.s.sing streams of gas.
One portion of the theory of electricity had been considerably developed by Cavendish; the application of mathematics to the theory of attractions, and hence to that of electricity, had been carried to a great degree of perfection by Laplace, Lagrange, Poisson, Green, and others. Faraday, however, could not satisfy himself with a mathematical theory based upon direct action at a distance, and he filled s.p.a.ce, as we have seen, with tubes of force pa.s.sing from one body to another whenever there existed any electrical action between them. These conceptions of Faraday were regarded with suspicion by mathematicians. Sir William Thomson was the first to look upon them with favour; and in 1846 he showed that electro-static force might be treated mathematically in the same way as the flow of heat; so that there are, at any rate, two methods by which the fundamental formulae of electro-statics can be deduced. But it is to Maxwell that mathematicians are indebted for a complete exposition of Faraday's views in their own language, and this was given in a paper wherein the phenomena of electro-statics were deduced as results of a stress in a medium which, as suggested by Newton and believed by Faraday, might well be that same medium which serves for the propagation of light; and ”the lines of force” were shown to correspond to an actual condition of the medium when under electrical stress. Maxwell, in fact, showed, not only that Faraday's lines formed a consistent system which would bear the most stringent mathematical a.n.a.lysis, but were more than a conventional system, and might correspond to a state of stress actually existing in the medium through which they pa.s.sed, and that a tension along these lines, accompanied by an equal pressure in every direction at right angles to them, would be consistent with the equilibrium of the medium, and explain, on mechanical principles, the observed phenomena. The greater part of this work he accomplished while an undergraduate at Cambridge. He showed, too, that Faraday's conceptions were equally applicable to the case of electro-magnetism, and that all the laws of the induction of currents might be concisely expressed in Faraday's language. Defining the positive direction through a circuit in which a current flows as the direction in which a right-handed screw would advance if rotating with the current, and the positive direction around a wire conveying a current as the direction in which a right-handed screw would rotate if advancing with the current, Maxwell pointed out that the lines of magnetic force due to an electric current always pa.s.s round it, or through its circuit, in the positive direction, and that, _whenever the number of lines of magnetic force pa.s.sing through a closed circuit is changed, there is an electro-motive force round the circuit represented by the rate of diminution of the number of lines of force which pa.s.s through the circuit in the positive direction_.
The words in italics form a complete statement of the laws regulating the production of currents by the motion of magnets or of other currents, or by the variation of other currents in the neighbourhood.