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By W. A. BENTLEY, W. J. HUMPHREYS
Dover Publications, Inc.Copyright © 1962 Dover Publications, Inc.
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THE SNOW CRYSTAL
Snow, the beautiful snow, as the raptured poet sang, winter's spotless downy blanket for forest and field, has ever challenged pen to describe, and brush to paint, its marvelous mass effects. Nor is the aesthetic urge of its very tiniest flake or smallest crystal that gently floats from heaven to earth any less compelling. It is even more insistent—doubly more—for it not only quickens that response to the dainty and the exquisite that makes us human, but equally arouses our desire to understand, our curiosity to know, the how and the why of this purest gem of surpassing beauty and of a myriad myriad forms. But it is so tiny, so fragile, and so evanescent save in the coldest of weather, that few, very few indeed, have come to know the snow crystal at first hand. All the rest of us must get our knowledge of this endless gallery of Nature's delicate tracery and jewel design through the careful drawings and faithful photographs, microphotographs, by that devoted few whose enthusiasm never wanes and whose patience never tires.
The first observer of all who have tried to convey graphically to others some idea of what the snow crystal is like was Olaus Magnus, Archbishop of Upsala. His curious book on natural phenomena, published at Rome in 1555, contains a quaint woodcut devoted to this purpose. However, the engraver certainly must have lost the good bishop's sketches and instructions, and then given to his greenest novice the honor of making this first of all cuts of the snow crystal. At any rate, but for the assurance of the legend, the intent of the picture would be wholly unknown for only one of the twenty-three small illustrations bears any resemblance to the thing it is supposed to portray. Nearly three centuries later, in 1820, the tedious art of depicting the snow crystal by careful drawings reached the stage of highest integrity in the work of the Arctic explorer, Scoresby, with a wonderful approach to accuracy.
Then came the photographic microscope that leaves nothing visible unrecorded, and adds nothing, whether for "art" or symmetry, that is not really present in some form or other. But only an occasional enthusiast worked on the snow crystal with this revealing instrument; and of these in turn the most persistent of all and the most successful of all is W. A. Bentley of Jericho, Vermont, who pursued and still pursues this search with the insistent ardor of the lover and the tireless patience of the scientist. And so it has come about that his careful search of snow after snow, winter after winter, for nearly half a century among the northernmost of the New England mountains has revealed not just a few crystal varieties, but many hundreds of forms, all based on a common hexagonal pattern—many, though, with their alternate sides more or less suppressed, even to the stage of being equilateral triangular. Not only did he find this great multiplicity of kinds, but also he skilfully obtained beautiful microphotographs of them, and thereby made it possible for others to share at leisure, and by the comfortable fireside, the joys that hour after hour bound him to his microscope and his camera in an ice-cold shed.
Finally, from time to time, during the last dozen years or more, the x-ray specialist has tried, by his subtle means, to coax the ice crystals to reveal its innermost secrets—to show what its smallest crystal unit is, and how shaped, and to throw off at last all disguise as to what exact type of crystallization it really belongs. Still the answers are a little evasive, especially those that concern the form of crystallization. But the specialist does not yet admit defeat in this particular—in fact he seldom needs to admit hopeless defeat about anything—and pretty soon he probably will be making good-sized ice crystals under controlled conditions and searchingly examining them with microscopes, polariscopes, piezoelectrometers, etching fluids, x-rays, and by every other means the circumstances may require and his ingenuity can devise. Then, if not before, will the snow crystal answer "yes" or "no" without qualification or equivocation, to the various questions about it we long have asked without response, and in all cases offer entirely satisfactory and unmistakable replies.
TAKING THE SNOW CRYSTALS'S PICTURE
Taking fine pictures of worth-while snow crystals requires good judgment, quick decision, speedy action, and great skill. They blow away and even melt away cringingly at the faintest breath, nor can they long endure the radiated heat from a nearby human body. Hence it is that skill and speed are of the very essence of success in photographing the snow crystal that but a moment is here then gone forever.
But how is it done? First you catch your snow crystal. This is conveniently done by holding a smooth black board, a foot or so square, a moment or two, or as long as necessary, in the falling snow. The catch is then taken under shelter, to keep it from being blown off the board or otherwise disturbed, where the light is good and the temperature that of outdoors. After a hasty inspection with a suitable magnifying glass a promising crystal, if one is found, is transferred carefully and with most delicate touch to a suitable glass plate—a microscope slide—with a small wooden splint, and there pressed down flat or brought into other proper position and made slightly to adhere to the glass by the gentle stroke of a small wing feather. After this it should be more minutely examined with a microscope to determine whether or not it is worthy of photographic preservation. If it seems to be worthless there is nothing to do, of course, but start all over again. When, however, a photograph of a crystal is to be obtained it obviously is necessary to take it with a photomicrograph camera, that is, a microscope fitted with a camera bellows and plate holder where the eyepiece normally is placed, or farther removed. The camera is turned toward the sky (clouds actually) either directly or through a window; then, or previously if more convenient, the crystal, adhering to the glass slide, is properly centered in front of a low-power, 1/2- to 3-inch microscope objective, and the focusing so adjusted as to give a picture of the desired size. The plate holder is then put in position, lens covered, slide of plate holder drawn, lens uncovered for time of exposure, lens covered again, and slide put back. One may proceed now, if one likes, similarly to make other exposures while the snow falls, and attend later to the developing, fixing, drying, labeling and storing of the negatives for future use. The light that comes through the crystal and by which its picture is made can be regulated by a suitable stop or round opening. In practice it appears to be best to make this stop rather small and to use a moderately "contrasty" photographic plate. The proper time of exposure, under these conditions, is around twenty seconds but varies so much with the brightness of the sky and with the equipment and plates used that each worker in this field must judge from his own experience what length of exposure to give.
Evidently the camera may be stood vertically, if so desired, and the light of the sky or cloud turned up through the crystal by a suitably placed mirror beneath it. In other respects, too, the details of the procedure may be varied. It is only necessary to work intelligently and patiently toward a definite end. It is this end-product that counts, not the minutiae of the process of obtaining it.
Since the snowflake or crystal is colorless and transparent it is obvious that the contrast between light and shade in its picture taken by transmitted light are owing to corresponding differences in the refraction, or bending, of the passing light, by the crystal. Most of these differences, in turn, are caused by minute, but nearly or quite symmetrically arranged, air cavities. Some, however, appear to be produced by tiny ice ridges, others perhaps by grooves, and still others apparently by crinkled water films incident to incipient melting. Even other causes presumably contribute in some cases to the final result, but if present at all they appear to be less effective than those just mentioned.
Since the ice crystal is transparent it is evident that a picture of it taken by transmitted light against a white background, such as a cloud, can not stand out in marked contrast with its surroundings. This defect can be largely remedied, as Mr. Bentley has done on duplicate negatives (not the originals) for the crystal pictures in this book, by patiently and with scrupulous care cutting the film away from all parts not actually covered by some portion of the crystal. When successfully done this greatly increases the beauty and artistic value of the pictures and at the same time leaves their scientific worth wholly unimpaired.
Occasionally some portion of an ice crystal, usually the central part, shows such wealth of detail as to justify especial enlargement, and the consequent exclusion of the other portions. A few, but very few, of the following illustrations were obtained that way.
SOME FACTS OF OBSERVATION
Perhaps the first and most important fact about the snow crystal that impresses itself on the careful student is the usual similarity of its general shape, while the second fact to be noted, also of great importance, is the endless variety in the details of its structure. These details have been the basis of several classifications of the crystal, but classifications that are useful only for descriptive purposes, since each merges imperceptibly into any other or may be connected with any other by imperceptible gradations.
For convenience in describing ice crystals, not for grouping them scientifically or according to their crystallographic pecularities, we may classify them as follows—may, not must, for it is only a personal choice and not a requirement by nature.
A. Hexagonal column, usually three to five times longer than thick.
1. Plane and with ends normal to side faces (see page 210).
2. With ends terminating in thin plates normal to the sides and of greater diameter than the column (see pages 210, 211).
3. With end plates and also one or more intermediate plates, normal to the column and dividing it into equal or nearly equal parts (not represented in these pictures, but well known).
4. With one end pyramidal, and one plane (see pages 210, 211).
5. With both ends pyramidal (not represented in these pictures).
B. Hexagonal right pyramid. Not represented here in any of its forms, but all known.
1. Right pyramid complete.
2. Right pyramid truncated.
3. Double or abutting pyramids complete.
4. Double or abutting pyramids truncated.
C. Hexagonal plate, ten times (or more) broader than thick.
1. Relatively large and plane (see pages 24-49).
2. Relatively large and with simple extensions at corners (see pages 50-72).
3. Relatively large and with elaborate extensions (see pages 82-118).
4. Relatively small, with long plane rays (see pages 150-151).
5. Relatively small with fern, or plume-like, extensions (see pages 165-195).
D. Triangular plates.
1. Plane (see pages 204-208).
2. With extensions (see pages 200-203).
E. Twelve-sided plates.
1. With extensions, six of one kind and alternately six of a different kind (see page 197).
As already stated, this is not a scientific or crystallographic classification, but just a convenient grouping according to general appearance, or to whether the major growth has been longitudinal, giving a column, or lateral, producing a plate, and to their modifications. The above scheme, therefore, is in no sense immutable, but subject to compression, expansion, or other change, as one's needs or wishes may suggest.
Perhaps three-fourths of all snow crystals belong to the tabular type, and, as is true of the crystals of other substances, few of any class are absolutely perfect.
CIRCUMSTANCES OF OCCURRENCE
These several classes of the ice crystal vary greatly in abundance. Furthermore, the relative amounts of each differ exceedingly with the conditions under which it is formed. Thus the simpler kinds, that is, the columns, or needles, as they usually are called, and the plane hexagonal plates, occur most frequently and in relatively far larger amounts at great altitudes where the temperature is very low, the amount of water vapor therefore quite small, and the growth of the crystals relatively slow. They usually are abundant in the highest of the clouds, the cirrus and the cirrostratus—thin wisps and veils through which the sun and moon can be seen distinctly. We know they occur there because these are the clouds in which halos, or the distant rings about the sun and moon, so frequently occur. And the halos give this information by virtue of the fact that they are produced by light that passes regularly, or uninterruptedly, through the crystals from one hexagonal face to the second beyond it; that is, in and out faces inclined to each other at an angle of 60 degrees, in the case of the 22-degree halo, the most common of all, and, in the case of the 46-degree halo, the next most frequent, in and out faces at right angles to each other, or side and end faces—courses in each case impossible through the more complex crystals owing to their numerous air cavities and other irregularities. These simple crystals, therefore, reach the earth in largest numbers in the western to northwestern (in the Northern Hemisphere, southwestern in the Southern) portion of a general snowstorm where the temperature is lowest and the falling snow fine and scanty; and also in very high latitudes.
The more complex crystals, on the other hand, especially those with branching extensions, occur at higher temperatures and, consequently, when the absolute humidity is relatively great. Hence they are most abundant in the front portion of the storm where the snow is heaviest and the clouds are lowest. Clouds of intermediate heights seem to give small, branching tabular crystals with solid, hexagonal centers. In other words, when the clouds are at intermediate levels the crystals tend to have intermediate forms. Whether this diversity in crystal form and detail is owing to differences in temperature, in absolute humidity, or in rate of growth does not appear to be definitely known. Neither does it appear to be an easy thing to determine, since at least a tendency to supersaturation is essential in any case for crystal growth, a condition that requires more and more humidity with increase of temperature. However, from the behavior of substances that readily can be crystallized in the laboratory under controlled conditions we infer that the differences in question presumably are owing chiefly, at least, to rate of growth—the small and simple crystals resulting from slow growth, the relatively complex varieties from comparatively rapid growth, such as would occur in air distinctly supersaturated with reference to ice, a condition that can obtain at any temperature considerably below the freezing point while it still is unsaturated with reference to liquid water. Presumably, too, the greater the supersaturation the faster is the growth and the more complex the resulting form.
The rarest of all ice-crystal forms appears to be the pyramidal, and, especially, the double pyramidal, or two pyramidal ends base to base, with or without a section of a column between them. This latter form, the double pyramidal, is not represented among Bentley's photographs, but it has been observed in polar regions by Dobrowolski (see Bibliography) and others, and is required to account for several halos of unusual radii, as explained in Physics of the Air by Humphreys. The circumstance or condition that determines the occurrence of this form in such relatively large percentage as to produce well defined halos is entirely unknown. It appears, however, to be very unusual, because the consequent halos, though fully recognized, are but rarely seen.
Reference was made above to the difference in the snow crystals that commonly occur in the front (or warm) and rear (or cold) sectors of a general snowstorm. It must not be supposed, however, that this difference is in any way a matter of location or of the surface temperature, humidity, or what not; it is owing wholly to the different conditions in the atmosphere where the crystals are formed, that is, and obviously, they depend on the conditions that obtain at the place where, and the time when, they come into being, and not on the conditions that may prevail somewhere else at some other time.
Excerpted from SNOW CRYSTALS by W. A. BENTLEY, W. J. HUMPHREYS. Copyright © 1962 Dover Publications, Inc.. Excerpted by permission of Dover Publications, Inc..
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