« PreviousContinue »
the second pane in other positions. It likewise loses the property of penetrating transparent bodies in particular positions, whilst it is freely transmitted by them in others. Light so modified, as to be incapable of reflection and transmission in certain directions, is said to be polarized. This name was originally adopted from an imaginary analogy in the arrangement of the particles of light, on the corpuscular doctrine, to the poles of a magnet, and is still retained in the undulatory theory.
Light may be polarized by reflection from any polished surface, and the same property is also imparted by refraction. It is proposed to explain these methods of polarizing light, to give a short account of its most remarkable properties, and to endeavour to describe a few of the splendid phenomena it exhibits.
If a brown tourmaline, which is a mineral generally crystallized in the form of a long prism, be cut longitudinally, that is, parallel to the axis of the prism, into plates about the thirtieth of an inch in thickness, and the surfaces polished, luminous objects may be seen through them, as through plates of coloured glass. The axis of each plate is, in its longitudinal section, parallel to the axes of the prism whence it was cut. If one of these plates be held perpendicularly between the eye and a candle, and turned slowly round in its own plane, no change will take place in the image of the candle; but if the plate be held in a fixed position, with its axis or longitudinal section vertical, when a second plate is interposed between it and the eye, parallel to the first, and turned slowly round in its own plane, a remarkable change will be found to have taken place in the nature of the light, for the image of the candle will vanish and appear alternately at every quarter revolution of the plate, varying through all degrees of brightness down to total, or almost total, evanescence, and then increasing again by the same degrees as it had before decreased. These changes depend upon the relative positions of the plates. When the longitudinal sections of the two plates are parallel, the brightness of the image is at its maximum; and when the axes of the sections cross at right angles, the image of the candle vanishes. Thus the light, in passing through the first plate of tourmaline, has acquired a property totally different from the direct light of the candle. The direct light would have penetrated the second plate equally well in all directions, whereas the refracted ray will only pass through it in particular positions, and is altogether incapable of penetrating it in others. The refracted ray is polarized in its passage through the first tourmaline, and experience shows that it never loses that property, unless when acted upon by a new substance. Thus one of the properties of polarized light is proved to be the incapability of passing through a plate of tourmaline perpendicular to it, in certain positions, and its ready transmission in other positions at right angles to the former.'
Mrs S. then proceeds to give the following explanation of the double refraction of light :—
Many other substances have the property of polarizing light. If a ray of light falls upon a transparent medium which has the same temperature, density, and structure throughout every part, as fluids, gasses,
glass, &c., and a few regularly crystallized minerals, it is refracted into a single pencil of light by the laws of ordinary refraction, according to which the ray, passing through the refracting surface from the object to the eye, never quits a plane perpendicular to that surface. Almost all other bodies, such as the greater number of crystallized minerals, animal and vegetable substances, gums, resins, jellies, and all solid bodies having unequal tensions, whether from unequal temperature or pressure, possess the property of doubling the image or appearance of an object seen through them in certain directions; because a ray of natural light falling upon them is refracted into two pencils which move with different velocities, and are more or less separated, according to the nature of the body and the direction of the incident ray. Iceland spar, a carbonate of lime, which, by its natural cleavage, may be split into the form of a rhombohedron, possesses this property in an eminent degree, as may be seen by pasting a piece of paper, with a large pin hole in it, on the side of the spar farthest from the eye. The hole will appear double when held to the light. One of these pencils is refracted according to the same law, as in glass or water, never quitting the plane perpendicular to the refracting surface, and therefore called the ordinary ray; but the other does quit that plane, being refracted according to a different and much more complicated law, and on that account is called the extraordinary ray. For the same reason, one image is called the ordinary, and the other the extraordinary image. When the spar is turned round in the same plane, the extraordinary image of the hole revolves about the ordinary image which remains fixed, both being equally bright. But if the spar be kept in one position, and viewed through a plate of tourmaline, it will be found that, as the tourmaline revolves, the images vary in their relative brightness-one increases in intensity till it arrives at a maximum, at the same time that the other diminishes till it vanishes, and so on alternately at each quarter revolution, proving both rays to be polarized; for in one position the tourmaline transmits the ordinary ray, and reflects the extraordinary, and after revolving 90°, the extraordinary ray is transmitted, and the ordinary ray is reflected. Thus another property of polarized light is, that it cannot be divided into two equal pencils by double refraction, in positions of the doubly refracting bodies, in which a ray of common light would be so divided.
Were tourmaline like other doubly refracting bodies, each of the transmitted rays would be double, but that mineral, when of a certain thickness, after separating the light into two polarized pencils, absorbs one of them, and consequently shows only one image of an object.'
The preceding exposition, though distinct and correct, would, in our opinion, be greatly improved by placing the account of the properties of the tourmaline after the explanation of double refraction, and showing, how by a mechanical change in the state of the surface in some crystals, such as calcareous spar, arragonite, &c., and in the interior constitution of other crystals, such as quartz, one of the two pencils formed by double refraction may be destroyed, as in the case of tourmaline.
Although Mrs Somerville does not profess to give a historical view of the sciences, yet she generally attaches the names of philosophers to their principal discoveries. There are several cases, however, throughout the volume, and particularly in the optical department, where this good practice has been overlooked, and where the entire labours of some eminent philosophers have been omitted. As an example of this, we may mention the optical discoveries of Dr Seebeck and Professor Mitscherlich. Although the one, had he been alive, and the other, who is still living, would have been indifferent to neglect in the chronicle of some churlish rival, they would have been proud to receive a leaf of laurel from the hands of our fair arbitress. The natural philosopher who might bear with equanimity the omission of his name from the more formal page of Professor Powell, would sink under its absence from the softer papyrus of Mrs Somerville. Even the rigid mathematician would prefer to the applause of Theon the smile of the accomplished Hypatia.
After discussing with much ability the subject of heat, and the general doctrines of meteorology, Mrs Somerville treats in succession of the popular sciences of electricity, galvanism, and magnetism, and of the new branches of magneto-electricity and thermo-electricity, which have had their origin in our own times. The sections which are devoted to these sciences will be perused with great interest by readers of all classes. They contain a brief notice of the splendid discoveries of Dr Faraday, and will, no doubt, incite the reader to seek in larger treatises for a more complete account of these great additions to modern physics.
In the perusal of these sections we have marked one passage on the subject of crystallizations, which seems to require some notice :
It had been observed that, when metallic solutions are subjected to galvanic action, a deposition of metal, generally in the form of minute crystals, takes place on the negative wire: by extending this principle, and employing a very feeble voltaic action, M. Becquerel has succeeded in forming crystals of a great proportion of the mineral substances precisely similar to those produced by nature. The electric state of metallic veins makes it possible that many natural crystals may have taken their form from the action of electricity bringing their ultimate particles, when in solution, within the narrow sphere of molecular attraction, already mentioned as the great agent in the formation of solids. Both light and motion favour crystallization. Crystals which form in different liquids are generally more abundant on the side of the jar exposed to the light; and it is a well-known fact that still water, cooled below 32°, starts into crystals of ice the instant it is agitated. Light and motion are intimately connected with electricity, which may therefore have some influence on the laws of aggregation; this is the more likely, as a feeble action
is alone necessary, provided it be continued for a sufficient time. Crystals formed rapidly are generally imperfect and soft, and M. Becquerel found that even years of constant voltaic action were necessary for the crystallization of some of the hard substances. If this law be general, how many ages may be required for the formation of a diamond!
The conjecture which is here proposed respecting the formation of the diamond, is, we suspect, more ingenious than sound. If the hardness of minerals were a function of their age, this important physical character in mineralogy would be useless. Quartz of recent formation is as hard as that which is found in the oldest rocks; and diamond itself, though the hardest of the gems, gives no indications of antiquity, either from its structure and physical properties, or from its locality in the crust of our globe. There is, besides, no reason to suppose, that the crystallizations in the bowels of the earth are dependent on voltaic influence. The mechanical action of electricity may promote crystallization, or increase the consolidation of previously aggregated molecules; but the nature and properties of a crystal are determined by general laws, of which electricity is not the arbiter. A perfect mineral species, undisturbed in its formation, derives its chemical and physical properties solely from the properties of its molecular elements. The presence of a small quantity of extraneous matter may affect the purity of its chemical constitution, without altering its form and general physical properties. The presence a greater quantity may keep its atoms at a distance, and change both its form and its structure; and powerful mechanical forces generated within the earth, whether they arise from electrical or from chemical causes, may produce still greater deviations from the type of the perfect mineral. But these are disturbing causes similar to those which produce deformity and monstrosity in the animal world; and we have no hesitation in asserting, that when a perfect mineral is recently formed out of the reach of secondary influence, its hardness will be just the same as if it had existed a thousand years.
With respect to diamond, we must speak with less confidence. There is every reason to believe, that this remarkable body is a soft substance coagulated by the slow action of corpuscular forces; but it is the only mineral which possesses this strange character, and cannot be taken into account in any speculations respecting the influence of time upon other minerals.
Mrs Somerville concludes her work with some popular and interesting notices on the subjects of the fixed stars, comets, and meteoric stones. On the last of these topics, the most curious, and still the most mysterious in physics, Mrs Somerville makes the following excellent observations:
'So numerous are the objects which meet our view in the heavens, that we cannot imagine a part of space where some light would not strike the eye;-innumerable stars, thousands of double and multiple systems, clusters in one blaze with their tens of thousands of stars, and the nebulæ amazing us by the strangeness of their forms and the incomprehensibility of their nature, till at last, from the imperfection of our senses, even these thin and airy phantoms vanish in the distance. If such remote bodies shine by reflected light, we should be unconscious of their existence; each star must then be a sun, and may be presumed to have its system of planets, satellites, and comets, like our own; and, for aught we know, myriads of bodies may be wandering in space unseen by us, of whose nature we can form no idea, and still less of the part they perform in the economy of the universe. Nor is this an unwarranted presumption: many such do come within the sphere of the earth's attraction, are ignited by the velocity with which they pass through the atmosphere, and are precipitated with great violence on the earth. The fall of meteoric stones is much more frequent than is generally believed; hardly a year passes without some instances occurring; and, if it be considered that only a small part of the earth is inhabited, it may be presumed that numbers fall in the ocean, or on the uninhabited part of the land, unseen by man. They are sometimes of great magnitude: the volume of several has exceeded that of the planet Ceres, which is about 70 miles in diameter. One which passed within 25 miles of us was estimated to weigh about 600,000 tons, and to move with a velocity of about 20 miles in a second, a fragment of it alone reached the earth. The obliquity of the descent of meteorites, the peculiar substances they are composed of, and the explosion accompanying their fall, show that they are foreign to our system. Luminous spots, altogether independent of the phases, have occasionally appeared on the dark part of the moon; these have been ascribed to the light arising from the eruption of volcanos; whence it has been supposed that meteorites have been projected from the moon by the impetus of volcanic eruption. It has even been computed that, if a stone were projected from the moon in a vertical line, with an initial velocity of 10,992 feet in a second, more than four times the velocity of a ball when first discharged from a cannon,-instead of falling back to the moon by the attraction of gravity, it would come within the sphere of the earth's attraction, and revolve about it like a satellite. These bodies, impelled either by the direction of the primitive impulse, or by the disturbing action of the sun, might ultimately penetrate the earth's atmosphere, and arrive at its surface. But, from whatever source meteoric stones may come, it seems highly probable that they have a common origin, from the uniformity-we may almost say identity-of their chemical composition.'
This opinion respecting the origin of meteoric stones, is certainly a very plausible one, and supported by many facts and analogies; but the recent discoveries of Dr, Fusinieri, which have not been noticed by Mrs Somerville, and which, we believe, have