PROFESSOR BICKERTON'S LECTURE

Star (Christchurch), Issue 2040, 21 September 1874, Page 3

PROFESSOR BICKERTON'S LECTURE

The sixth and concluding lecture of the series, by Profeaßor Bickerton, on " Matter and Energy," waa delivered at the Oddfellows' Hall, on Thursday evening laßfc, before a very large audience. Professor Bickerton was loudly applauded on coming forward; He said : Ladies and gentlemen, our lasfc lecture treated of the development of voltaic elecfcricty, and the various forms of energy into which voltaic electricity can be converted. Thus we saw that as voltaic electricity waa generated by chemical action, so it was capable of undoing chemical compounds We then traced out the pr duction of magnetism by voltaic electricity, and I showed you some of the applications of the magnetism so produced ; and afterwards we spoke of the conversion of voltaio elect ricity into heat and light. We saw that to produce heat a quantity of voltaic elecfcricty had to pass through a wire in which there was considerable resistance to its passage, and we saw that if this quantity of voltaic electricity was very great, light as well as heat was developed. I afterwards showed you the production of light in socalled induction tubes, and then the passage of the voltaic current between two pieces of carbon producing the electric light, properly so called. This evening I have to speak of radient energy — fchafc is, energy which flows out from centres. Afc the present moment light is passing from these gas burners and illuminating everything around, and we call this emission of light and heat from these burners " radiant energy." But the best way of showing radiant energy is by means of the electric lamp. Unfortunately, it has had to be somewhat awkwardly placed this evening, and I shall have to go to where ifc stands to show you my experiments with fche electric lamp, which I may tell you are experiments in every sense of tho word, for time has not permitted our trying anything before hand. I allow the electricity to pass between the two carbons, and inßtantly a brilliant light is produced. Iclose the door of the lantern, and take the cover off the nozzle of the lantern, and instantly a brilliant cone of light ia seen stretching across the room. The track of the beam is made visible by the particles of duet in the air, which are illuminated by the bright light. There i 8 a radiation of heat as well as light. On placing my hand into the cone of light, a distinct warmth is felt. If I place a little gun cotton iv the focus of the reflector, ib will, perhaps, be ignited. It has inflamed, although you see ifc is a considerable distance from the lamp. I can show this radiation of heat in another way. This red paper is coloured with iodide of mercury, which has the property of turning yellow when heated. I place ifc in front of fche lamp, and soon a yellow spot announces fchat heathas passed from the lamp. The paper is smoking, and if I hold it long enough, we shall, perhaps, ignite it. The smoke iB increasing/ and now a bright spot shows the paper ia ignited, and continues to burn after taking it from the light. I have already shown you thafc heat may be carried away by conduction and convection. We see here, it can alao be carried away by radiation. I must now proceed fco shew you some moro of the properties of thia radiant heat j but, before doing ao, I will try and explain to you a littlo of the theory of light and heat. Sir Isaac Newton believed that when the light from the sun and other luminous bodies reached the earth, ifc was ai_ emission of a species of matter coming from the sun. He believed ifc was an immense number of little particles being sent; out from fche sun, or a candle, or any other luminous body, which, striking the eye, produced the sensation of light. In explaining his theory, he assumed thafc light travelled raster in glass than in air. Sinco his time, a number of experiments have been made which demonstrate exactly the opposite conclusion to that arrived afc by Sir Isaac Newton, and consequently a now theory of light has been produced, and this is called fche undulatory theory, which I will try to explain to you. We have here a number of solid bodies, which we know can be converted into liquids and then rendered gaseoua, and we know also that we exist in an atmosphere of gas. By a number of experiments, in the course of time, we are able „very clearly to realise the exiafcence of fche air, bufc fche main theory of light supposes , something still more delicate than the air — that, in fact, when the varioua parts of a metal become vapour or gaa, all the spaces between these particles are filled up with a substance called ether, which is supposed to permeate every substance, and this substance ia capable of being pufc into a state of wave motion, and this wave motion acting on fche eye produces the effect of light ; so fchafc instead of light and heat being any solid subsfcanoe ifc ia a variety of motion. The existence of this wave motion in the air, when sound is produced, is very easily illustrated. The medium which carries fche sound of fche voice to you is the air, and I think I can show you that when I speak, I really move the air. I have here a gas burner, which, as I light ifc, produces a very long thin flame. I make a sound and

you see ifc instantly shortens to one third i's length. The flame is like many j>eop'e, it :.* very sensitive to a hiss. If aome one in Ihe gallery would oblige me with a hiss I thin. our flame will bow to ifc. Thank you. You see that no matter how distant the sound i 9 the flame responds. So we see tliat sound is convoyed in some way by the air, and a dis' fc sound will cause tho fUme to move. ' -i know when you throw a atone into wah .. circle of wave? are produced which quick iy spread to the edge of the pond. The water <>i the centre does nofc movo to the outside, ifc is only fche impulse that moves, and -ao wifch sound : a persona noise creates a disturbance of the air, and thia disturbance is propagated aa a wave, and the flame makes this diaturbance evident to you, it reaponds most readily to thafc most unsatisfactory call, a hiss (Laughter). Here, then, this flame ia moved hy what is called wave motion. So in the same way with light, only instead of bavjpg fche wave motion which ia produced in the air, it is a wave motion produced in something very much more delicate than the air, namely the ether I have spoken of. This movemcrt constitutes both radiant heat and radiant light, the only difference being that in the case of radiant heat the rays are stronger and of a larger kind, while those of radiant light are more rapid and shorter. These waves of light and heat proceed in perfectly straight lines aa long aa they are in a uniform medium. Thua, if I allow the light from fche electric lamp to pass through a small hole, you see how perfectly straight is the beam of light produced. This rectilinear propagation of light is the cause of many interesting phenomena. The images of objects produced by small orifices . c a good illustration. If a small hole be made through a shutter into a dark room, an: nverted picture of what is outside is produced. Thus, if a churoh be outside, a beam of light ' would proceed from the tip of the Bteeple ■ through the hole and fall upon the lower part of the wall. So a beam from the bottom falls on the upper part of tho wall ; and, in tho same way, light proceeds from every part of the church cross at the hole and forms an inverted pijfcure on fche wall. I can illustrate this by the electric lamp. You saw last week the form of tho carbons between which tbe electric spark was passing. The nozzle of tb lantern ia covered wifch tinfoil. I prick a hole through with a needle, and instantly an imafjo of light appears on the screen. I prick another hole and another image appears, and £. as I make holes, at last the screen is covered with theimages. This experiment proves very clearly ihe fact that light proceeds in straight lines. When a beam of light falls upon a white surface, such as paper, the light is sent off again in "every direction, but if ifc falla upon a look-ing-glass ifc ia sent off in one direction only. Thus the beam from the electric lamp falling upon tbis looking-glass, and you sco . leaves the glass and passes across the room e.i a straight beam. This rebound of the light; from a polished surface is called reflection. If I move the glass, it is only those immediately in the line of the beam who can see the electric light reflected from the glass. If I hold the glass at an angle of half a right; angle, the horizontal beam striking it is mado quite vertical. Thus we see that the angle afc which light leaves a mirror is the same as thut at which it falls upon it j this part is usually expressed by saying the angle of incidence and reflection are equal. When I hold this candlo in front of a mirror, an image of the candle :■-> Been, through the rays that proceed from ti..candle striking upon the mirror being reflect: ... and the reflected ray falling upon the ray. If two mirrors bo used at a great angle, two images ara seen. If the angle between the mirrors be small, you see an immente number of images, from the fact that the candle is reflected backwards and forwards from mirror to mirror. This is the prinoiple of the kaleidoscope. I have a kaleidescope in the lantern, and you see thafc although fche light on the screen is pretty uniform it is divided into eight spaces. If I place a small loop in the lantern we have eight distinct images. I coil up a piece of wire, and on placing it in tho lantern a moii beautiful geometrical pattern shows itae!' ; , altering into every conceivable shape and '' sign. Were a drawing master here, he wo.. Jewish for theso splendid curves as examples jv-.' his pupils to draw from. I place in the lantern a piece of painted glass, arid on moving it about a most exquisite changing series oi patterns are produced. The patterns produced by the, kaleidescope are frequently used in design. I mußt now speak of another property of light called refraction. Whenever a beam of light from a rare medium, such as air, falls obliquely upon a dense medium, such as water, the beam is bent out of its course. i Thus when a beam of light falls upon thia prism, which is a triangular piece of glass, ■thei beam is bent, and proceeds in a nerr direction. . This , bending of the beam is called refraction. If instead of a prism of gtess aprismof bi-sulphideof carbon be uaed,anofcher phenomenon shows itself at the same time. The light is nofc only bent but is also coloured. The beam of light lam using, when it meets with no obstacle, proceeds directly across fche room . bufc in placing the prism in its path, ifc is bent so as to fall upon the screen placed in front of the platform, and there, instead of a narrow upright slip of light, it forms a long horizontal band of va.ious colours. You will notice fchafc fche red is least bent out of the straight lino, then comes orange, yellow, green, blue, and violefc ; bo you will remember thafc the red is tbe least bent and the violet the moat bent. By this experiment we see tbat white light contains all theso colours, and the prism simply splits it up into its constituents. If I place a aecond prism iv fche opposite direction to tho first, all the colours are brought together again, and white light is produced. The eyo retains an impression ifor a short time and tbis enables me to show this fact very clearly. In the lantern is a disc of glass painted in segments of different colours. An image .of the glasa i.; now on the screen ; the glass is made to revolve, and you see aa it does so the colours entirely disappear and white light ia produced, thus showing in a most striking manner fche compound nature of white light, Ifc is possible to produce a light containing one colour only, thus by burning alcohol with a little common salt a pure yellow flame ia produced. I ignite thia piece of magnesium wire, which produces a beautiful white bght, and you Bee tho colours on this piece of piper most brilliantly. I extinguish the magnesium, and hold tho coloured paper behind th.; yellow flame produced by the sodium of fclr. common salt, and all the colours except fch' yellow are losfc. They only Bhow difierenshades, bufc no colour is visible ; fche red _l quite black, so is the blue. The red of tha paper can only reflect red, the blue only 51ue, and so the yellow light falling on them is quite losfc, and blackness and the absence of colour is fcho result. If I hold this variously coloured paper in the spectrum, you see in fcho red part the red is visible, all other colours are losfc. In the orange, the orange is distinct,

in fche green the green, and in Die blue the blue. On throwing fche electric light on the paper all its colours are seen, so are the colours on the large diagram at the b tck of the screon. I hope it is now clear to jou, that, in order to be able to see all kinda of colours at once, we must have white light. What a wretched appearance a ball, or a fl >wer show, would present if a sodium fhme waa the only means of illumination. Cj lour in a body then is due to the power ifc possesses of reflecting the particular part of white light fchci produces that especial colour, and of destroying fche other rays. I will now try to explain to you the theory that ia ÜBed to explain the nature of white light, and the colour of the spectrum. Light, I have told you, is believed to be due to waves in a delicate impenetrable substance called ether. White light consists of waves of every length between 36,000 fco fche inch aud 64,000 fco the inch. The red waves are the longest, the orange next, and so on to the violet waves, which are the shortest. The whole of the white light travels at the same velocity, and so the short waves have to vibrate faster in order to keep up with the others. I will try and make the matter clearer by a simile. Let us imagine a crowd "of people of various sizes, all going in a straight lino fco the same place, fchey all .keep together, and hence the little ones have to make much quicker steps in order to keep up with the tall ones, who are striding along at a leisurable pace. This crowd represents our white light, the tall ones are red, the very short ones are violet, and all intermediate lengths are of their appropriate colours. Such a onwd as this, afc a distance, would look white. Let us now try and conceive of the production of our spectrum. Suppose, as this crowd walks along the plain, an impulse turns them all partly round, fche strong redl one it only turns a little, while the weak violefc ifc turns much more. If all now proceed straight on in the direction they now. face, instead of keeping together they gradually spread out into a long line, tho red at one end, the violefc at the other, and the other colours filling the space between them ; producing, in fact, jusfc the appearance our spectrum does on the screen. We will now try and apply our simile to spectrum analysis. If we place between the carbons of our electric lamp a piece . of sodium, an intensely yellow light is produced, and on the screen, instead of a long continuous spectrum, a single upright band of yellow is seen. Sodium, in burning produces waves of one length only. So if we consider our brain of light to be a crowd, as this crowd consists of persons of exactly the same Bize, bo when they receive the turning impulse, all are turned exactly the same amount, so on proceeding in the new direction, all keep together instead of being opened out into a line. So with our sodium light, the waves are all of equal power, and so our power although it turns them out of fcheir original direction, turns all alike, and so they keep together, and produce a narrow strip of yellow instead of a broad spectrum of many colours. On placing other metals in the lamp, different bands are Been ; some metals producing a large number of bands, and so by burning any metal and examining the bauds of light, the nature of the metal iB as surely pointed out as though its name we-'e written down for the analyst s instruction ; so delioate is this system, that it has been proved that a quantity of sodium less than a millionth part of a grain is able to be detected. If, whilst the spectrum iB on the screen, I place a piece of red glasß in the path of the beam, all the speotrum but the red is destroyed. The red glass is a kind of sieve that will only allow red rays to pass. If I place blue glass instead, the blue of the spectrum only appears; in both these cases the glass absorbs all the rays of the spectrum that disappear from the screen and allows the others to pass th.ough. Thia absorption ia much more characteristic in the case coloured chemical solutions than substances absorbing only particular partsof the spectrum, and eacii substance absorbing different parts. This is especially the case with blood ; for this substance the spectroscope is a most certain and delicate means of showing its presence. We have seen the bright yellow band produced by the spectrum of sodium. If instead of burning ifc in fche lamp I produce a powerful spectrum from tbe lamp and place ifc in fche path of the beam, a flame containing sodium instead of a bright yellow band, I get a speotrum from whioh that particular band iB absent. The soda flame absorbs the rays from the electrio lamp whioh it of itself gives out. It takes up those waves of yellow, and instead of sending them on in a straight line, it gives them out ! in every direction, so that the beam of white light proceeds straight on to the prism, while the particular yellow waves of fche sodium are so scattered over the room that so small a number reach the screen as to give to the eye an impression of blackness. For a lung time, ifc has been known that the Bpecfcrum produced by . the sun's light ws _ crossed by an immense number oc black lin .3; these line 3 were found to be exactly coincident with the bright bands produced by burning metals. The explanation now given of those black lines is that the sun is like the electrio lamp — a centre of intonse luminosity. This luminous centre is surrounded by metals in a state of vapour, and these vapours absorb the light and so produces tbe dark spaces. There is scarcely a doubt bufc thiß is really the caie, so that in tho spectre-cope we have an instrument that is not only capable of analysing the subtances on the earth, but, by its means, we can tell the constitution of the sun, the distant stars, and the various selfluminous bodies wbich are scattered throughout fcho entire universe. Ifc has shown us that all those distant bodies are composed of elements such as we find on our own earth. This gives a great probability to the nebular theory of the formation of the universe, which supposes the whole of the heavenly bodies to have had one common origin. The speotrum whioh extends itself along this screen, con6i3fc3, aa I bave stated, of waves of length ranging between 36,000 and 64,000 to the inch j theso aro visible to tho eye. There are other waves, both longer and shorter, the existence of which can be proved by various ways, but which are not visible to the eye. Those rays beyond the red are heat rays, and in the spectrum of this electric lamp seven times as much heat falls beyond the visible spectrum as upon ifc. At the other end of the spectrum, that is beyond the violefc, fche spectrum also extends, and these are fche very short waves. These waves have nofc much energy, but they have the power of decomposing many chemical substances. Wo have seen fchafc fche rays from the electric lamp are capable of igniting a piece of paper. Most of fchafc power is contained in waves longer fchan the red. We have seen that coloured glasß will only allow certain waves to pass. You can easily conceive there are some substances which will allow all fche visible waves of the spectrum to pass, bufc stop j the long heat rays. There are severaTßja^ i substances, but tho moat powerful isp_M*tmon

alum. These bodies, from their property of stopping the heat rays, are called athermic borlies. In the same way we have substances whicli will not allow light waves to pass, bufc allow fche long heat waves to pass readily. The best representation of this family is iodine. These smbstancea are called diathermic, bodies. Thus by placing a solution of iodiue before fche electric lamp the whole of the light may be cut off, but heat will pass readily and maybe brought to a focus, which although quifc9 invisible is very powerful ; thus a piece of platinum foil is instantly made red hot and a cigar may be ignited in it. This production of dark-heat foci, iB called caloresence. I hope now you clearly picture to yourself this spectrum wifch its waves of various lengths, that gives us the coloured part ; also the extension beyond the red end of long heat iays, and on the violet of short chemical rays. We will try and make out more clearly the action that occurs when the waves at the short end of the spectrum decomposes chemical substances. We saw the rays from the lamp ignite the paper, and I told you it was principally the long waves that caused ita ignition. You know that in order to ignite the paper its molecules must be put into a state of intense vibration, the long powerful heat waves are competent to do thafc, bufc not so the short chemical waves They have no power to move the molecules as a whole ; but they can move the little atoms of which the molecules are built up. These atoms keep vibrating more and more as wave after wave posses it, and at length the energy of its vibration loosens it from its fellows and it is Bet free. It is thus the light decomposes the salts of silvei* used in photography, and so prints a picture of whatever fche light ia passing from. We have yet to speak of another effect Miese waves, called fluorescence, so called from its first effects being noticed in fluor spar. Fluor spar, when heated to a very low temperature, will emit light. I have here a number of tubes containing various fluorescent substances. On holding it in the light from the electric lamp, the tubes are seen to become self-luminous, and remain so for a considerable time. So if I allow the electric apark to pass through this tube afterwards the substance glows with a steady green light. In these cases it is evident that the particles of the substancea are Bet vibrating by the energy of. tho light-waves falling on them, and this vibration continues some time after the cause is removed. A large number of substances show this effect in a greater or lesser degree. This glass (of whioh this tube is made), coloured by the oxide of uranium, possesses it, but its duration iB so short that you cannot see it when I allow a spark to pass, but if the dish carrying the tube be mode to revolve, each spark produces a flash of light and leaveß a yellow trail behind, showing that the glass continues the glow after the flash has passed. If I send the spark through another tube not fluorescent, you see a star of images of the tube on a black background, the absence of luminosity showing that the spark itself is not fluorescent. We have traced many transformations of energy, and I must now say a few words about a theory first announced by Sir W. Thompson, and called the dissipation of energy. AZb far as we know, whenever heat is converted into work, heat from a high temperature is taken to a lower temperature, and so passes off. The sun sends off heat rays, and all these rays are gradually dissipated into space, so that although energy is not capable of being annihilated it is capable of being converted into heat at one temperature, and so rendered unavailable for doing work. So that the present views of scientific men clearly point to an end of the present state of things when all substances are reduced to one common temperature, and the entire energy of the. univeree brought to an unavailable form as far as concerns any purposes it at present serves. Ladieß and gentlemen, I have to thank you very sincerely for the patience and attention wifch which you have listened to me throughout thiß course of lectures. I fear the experiments which I have made have not as a wholo been bo clearly seen as I could desire, but you can easily see a long room like this is not at all adapted for scientific illustrations. I never anticipated that these lectures would be attended by such large numbers, and our only reason for taking this large hall was because we could not get a Bmaller one. With a sma'l audience, tho apparatus I had would have been sufficient, but so many persons who were at the baok of the hall told me they were unable to see, that I have tried to remedy this as far as possible. We have, therefore, made much of the apparatus we have used in the intervals between the lectures. Had I known this beforehand, and also the unsatisfactory state in whioh much of our apparatus was in, I shonld not have commenced the course until a few weeks later, as it has been only with the greatest difficulty we could prepare the experiments within the week. I beg to acknowledge the great assistance I have received from Mr Noble, the philosophical instrument maker, who has worked many times into the smali hours of the night in order to finish the apparatus in time, bufc for his assistance we would nofc have accomplished a great number of the experiments. (Applause.) I I have been somewhat blamed for taking you over so large an amount of ground. Doubtless, it would have been easier t_» have given a course of lectures on light, heat, chemistry, or eleotricity. But the sciences are so blended together that in order to understand any one of them it requires some knowledge of the whole. Thus, when we speak of heat, we require to know light to understand it. Volt Jo electricity requires a fair chemical knowledge, and ohemisfcry some knowledge bofch of heat and electricity. So I thought I would bring before you a kind of skelet m of the sciences to show you their mutual relations, and then on come future occasion we might go into the details of each particular part of the skeleton, and ifc would nofc be in perfect ignorance of the whole thafc you worked at the clothing of any particular part. I have tried in this course fco give you a clear idea of the indestructibility of matter and energy ; how matter is endowed with certain forces ; how these forces praduce the various kinds of energy ; and fche convertibility of these d iff eren trior iffiT of energy one into the other. Before concluding, I must warn you that popular lectures will never- serve the purpose of regular study. To an earnest student, popular lectures may, hy their experimental illustrations, give a reality to hii ideas, _whicJi_J__joks alone cannot. Popular lectures may also give an idea of the range of science and the vast field of ita usefulness ; but alone, they can give bufc a mere " smattering" of knowledge. Ifc is only by attending a regular cov^o of study fchafc any real knowledge cp<i be gained ; like all valuable things, H^jnly by hard work it oan be obtained, more, ladies and gentlemen, allow me to : you for the patience and attention you hav^ displayed throughout the series. (Loud i applau&v) v ""