all quarters is bringing us nearer to this ideal; and there is not an approaching reformation,' be it what it may, whose advent has not been approximately hastened by the ceaseless operation of our free press in the diffusion of knowledge, the stimulation of mental activity, and the development of a generous zeal for the advancement of the common good. SOUND AND SENSE. 1. Sound. A Course of Eight Lectures delivered at the Royal Institution of Great Britain. By John Tyndall, LL.D., F.R.S., &c. London: Longman. 1867. 2. Heat Considered as a Mode of Motion. By John Tyndall, F.R.S. London: Longman. 1865. 3. Ganot's Physics: Experimental and Applied. Translated by E. Atkinson, Ph.D., F.C.S. London: Longman. 1867. 4. Lectures on the Science of Language. By Max Müller, M.A. Second series. London: Longman. 1864. HE atmosphere has a delicate sensibility. It receives and communicates, but never creates. It is full of subtle circulations, invisible poems, and grand architectures. It rises, and falls, and flows under the sweet ministrations of sun and moon, with a trembling love and a consenting power. It wraps the earth with chemistries that match the magic of love that bathes the universe. The autumn leaf sings out through it its tender elegy, as it dies downward to the earth, and the shiver of tiny moss-spears and the undulations of fern leaves are all as surely written out in airy rhythms as are the mad dances of the sea, the hurtlings of the avalanche, or the crashes of the storm. Without it there would be no life, no language, no melody. As the air attenuates, so do all sounds become fainter and fainter, until there may be a limit where, if our earthly human organisms could exist, we could no longer hear a sound or a syllable, and all things would be bathed with silence as with a sun that cast no shadow. Wonderful as the air may be as a vast orchestra, possible and actual, a human being is still more wonderful. He is variously equipped to meet its marvellous conditions. In his five senses are exerted separate methods of analysis and dissection, and where one fails the other steps in to take its place. He is thus the interpreter of the powers of the air. The retina of the eye is hit by wave after wave, and the undulations announce themselves to his brain as light and colour. Motion, translated through the invisible particles of the air, collects itself in his auditory apparatus, and becomes sound. Minute, invisible gases and particles impinge upon his olfactory nerve, and are known as odours, and the molecular motions of solid and other bodies, imprinting themselves upon the air, are recognisable as heat. Taste alone seems, yet only seems, outside the pale of the other senses in not being the result of direct motion of any kind; but it is only because we cannot, so to speak, approach its analysis from the outside, there being simply the wave-motion made in the mouth, promoting direct contact with branching nerves and glands. Taste cannot, however, be fairly considered as exceptional, any more than can mere touch. As Professor Tyndall himself observes, it is the motion excited by sugar in the nerves of taste which, transmitted to the brain, produces the sensation of sweetness, while bitterness is the result of the motion produced by aloes.' All sense is, therefore, a sort of motion, though all motion is not sound. The motion may be a translation, or a creation, it matters little. Were our ears keener we might listen to the noise of the undulations that we call light, and if our eyes were more exalted in power, we might see the rhythmic waves that compose a musical note, as we might watch some sweet odour diffusing itself in the air. Our senses, however, are not interchangeable in their action, though it is now quite possible, by the aid of the electric lamp, to make a musical vibration visible to the eye, and by other means, as Trevelyan's Rocker, to make audible the atomic oscillations of heated bodies. When, however, we have done so much, we have not explained the mystery of sensation, but simply driven it further back. All nerve-structure resolves itself into cell and fibre, and yet why similar constituents should hear, or see, or feel, or smell, or taste, even when all are affected by a similar motion of their particles, accordingly as they are situate in different parts of the body, is as much a mystery as ever. The physical explanation is easy, but the chemical one is not quite so simple. Even analogy does not help us much. Ozone and oxygen are intimately related, nay, are held to be the same thing in different states, but in power of radiating heat the former exceeds the latter a hundred thousand times, just as the nerveforce that constitutes hearing is like in character to that which makes sight actual, whilst eleven octaves are possible to the ear, and very little over one to the eye. In view of this power in the arrangement of simple atoms, Professor Tyndall asks, respecting water, whether its molecule, from which its vast radiant power is derived, may not be a molecule of molecules, the chemical formula stamping only a single member of the group?'* We know not what may yet be possible to microscopic research, but it may hereafter be found that the laws which regulate the propagation of sound-waves and gaseous particles correspond to the structural principle of different nerves. For instance, sound spreads itself equally in all directions where there is no resistance from objects, currents, or varying densities; odours and gaseous particles fly in straight lines, and ether-particles, excited by radiant heat, move in tranverse undulations, as may be seen in the shimmer of the air from a housetop or a tree in the full blaze of a summer's day. It may be a fancy now, though it may become fact hereafter, that the position the cells hold to each other in the grey substance of the brain and spinal cord, and the composite arrangement of the layers of the white, or fibrinous matter, really occasion the different character of one sense from another. In the action of all, what we know of the physics and chemistry of sound may help us. The nerves themselves do not move, any more than the air-particles do, en masse, under the vibrations that form sound. Each molecule simply makes a slight excursion to and fro, though the pulse, or the sensation, is rapidly translated through all. In both, heat plays an important part; thus, a sound-wave consists of two states, a compression and a relaxation, or, in scientific words, a condensation and a rarefaction, the distance between two condensations constituting a sonorous wave, heat being developed in the squeezing together of the air particles in the swell of the wave. So, chemically speaking, is heat developed by, and necessary to, all human sensation. If the brain be frozen, as Dr. Richardson has shown, either in sections or as a whole, its power to will and feel is gone or modified, though life goes on the same externally, and light may strike upon the eye, and sound pulse in the ear. With the application of warmth, intelligence and sensation return, sometimes sensation preceding motion, and sometimes motion sensation.† Thought and sensation are thus attended by a sort of oxidation produced by composite causes, of which heat is the resultant force, heat being, as every one now knows, molecular motion. Thus, as Professor Bain aptly observes, there is, although *Fortnightly Review,' Vol. IV., p. 4, Note 1. + Physics of the Brain.' 'Popular Science Review,' No. 25, Oct., 1867. we may not have the power to fix it, a sensational equivalent of heat, of food, of exercise, of sound, of light; there is a definite change of feeling, an accession of pleasure or pain, corresponding to a rise of temperature in the air of 10 degs., 20 degs., or 30 degs., and so with regard to every other agent operating upon the human sensibility; there is in each set of circumstances a sensational equivalent of alcohol, of odours, of music, of spectacle.'* This translation of force into the material conditions of sense from without and within, may be called either heat or motion, since it is both, but it no more exhausts the nature of thought and volition than dissection is a re-creation. Will itself is not motion, though so far as we can reach its beginning, chemically, it is so, and issues in it, any more than hearing is the half of its process that terminates when the helix is passed and Corti's organ is reached. We may have perfected air analysis by a better understanding of sensational and intellectual conductivity, if we may use an electrical term, but we are still baffled by the creation and registry of power. It is no doubt inspiriting to have got thus far, but we have only penetrated one wall of being to find the citadel of the soul still within, and within, yet flashing out through all. The mistake is in assuming, on the one hand, that when we have mastered the law of a process we have grasped everything from its inception to its outcome, or in decrying as materialists those who persist in endeavouring to penetrate through the veil of sense, and refuse to be satisfied with the mere use and handling of such counters as sensation, volition, or instinct. us. There are many other analogies between sound and sense that will come out as we proceed. A sonorous wave, as we have already stated, consists of two parts, a condensation and a rarefaction. It cannot, therefore, be heard in vacuo, and is variously affected by the temperature and density of the air, or the structure of any medium through which it may reach As a rule, the intensity of a sound depends on the density of the air in which it is generated, and not upon the character of the air in which it is heard, though enfeeblement necessarily takes place if a sound passes from a light body to a heavy one. Thus, a cannon fired on Mont Blanc would be heard with the same intensity by a person standing on the bridge at Chamouni and another on the top of the Aiguille Verte, though in one case it would pass upward through rarefied air, and in the other downward through denser air. *On the Correlation of Force in its Bearing on Mind.' 'Macmillan's Magazine,' September, 1867. The transference of sound from air through glass is, perhaps, the best illustration we can get of its enfeeblement, though it may also be noticed where the atmosphere is not homogeneous. Humboldt thus explains the fact that the noise of the falls of the Orinoco was heard much more plainly in a certain position of the Antures by night than by day. During the day the radiant heat from the bare rocks between the observer and the falls presented a constant change of density to the passage of the sound. Peals of thunder are not able to crash upon us as they would, and very fortunately, too, from the same mixed character of the air. 'From the same cause battles have raged and been lost, within a short distance of the reserves of a defeated army, while they were waiting for the sound of artillery to call them to the scene of action.' The influence of temperature and density upon velocity is very considerable; and Laplace's correction of Newton's formula is very interesting to the student, but it needs only to be stated here verbally, in conjunction with Mariotte's law. A change of density in the air does not affect the velocity of sound unless there be a change of temperature. We frequently hear a curious illustration of this. A daily time-gun sounds to us on a fine day as though it were fired a long way off; but when the air is moist and very warm it seems to burst immediately outside the window. No doubt the particles of water in the air have raised its power of conduction, along with a sensible increase of warmth, for in a cold fog the sound is heard in the ordinary manner. Elasticity and density neutralise each other where the temperature is the same, as the illustration of the gun fired upon Mont Blanc seems to show; but for every rise in temperature of a degree centigrade the velocity of sound is augmented two feet, its ordinary velocity being 1,090 feet second in air of the temperature 0° C. Comparative velocities are very interesting. The lowest is in carbonic acid at 858 feet per second, and the highest in hydrogen at 4,164. In metals, iron is most singular. Velocity, instead of diminishing as, with the exception of silver, it does in most metals, by increase of temperature, rises sensibly in iron until a point is passed beyond 100°, when it begins to fall again. M. Biot found that in sounds transmitted through the water pipes of Paris, two sounds reached the ear in succession; the first from the iron and the second from the air. Molecular structure affects transmission very sensibly. Homogeneous bodies transmit it equally in all directions; but different structures, as in trees or crystals, act very curiously. Wood, as Savart has shown, has three unequal axes of calorific, and hence of sonorous conduction-along the fibre-across the |