. Compendium of meteorology. Meteorology. THE FORMATION OF ICE CRYSTALS 209 touched freezes, while the streamer disappears; the behavior of this streamer which relays the freezing action is beautifully demonstrated by slow-motion pic- tures, one frame of which is shown in Fig. 3. This frame shows the instant when the droplet marked A becomes frozen by a streamer from the already frozen droplet B. There are innumerable variations in the shape of window hoars observable in nature, for example, spiral patterns, odd arabesque designs, snow-like forms, etc. The ordinary glass plate is always covere
. Compendium of meteorology. Meteorology. THE FORMATION OF ICE CRYSTALS 209 touched freezes, while the streamer disappears; the behavior of this streamer which relays the freezing action is beautifully demonstrated by slow-motion pic- tures, one frame of which is shown in Fig. 3. This frame shows the instant when the droplet marked A becomes frozen by a streamer from the already frozen droplet B. There are innumerable variations in the shape of window hoars observable in nature, for example, spiral patterns, odd arabesque designs, snow-like forms, etc. The ordinary glass plate is always covered with an in- visible film of some organic substance. It is found that the combination of the effect of this invisible film and the atomic nature of ice gives rise to the variation observed in the patterns. When the glass surface is chemically clean, crystallization takes place very slowly. Even when the crystallization is almost complete, the hoar crystal is very thin and greatly distorted. The effect of an adsorbed film is very well demonstrated by exposing the glass plate to alcohol vapor for a short time, so that the surface is covered with an invisible film of alcohol molecules. Alcohol has a strong affinity with water, and the growth of the ice crystal may be expected to suffer a marked deformation. The results of such an experiment are as might be expected. One example is shown in Fig. 4. The opposite effect can be observed on a glass plate covered with a thin, invisible film of paraffin wax. Since the water is repelled by the paraffin film, crystal- lization must be free from the effect of the surface. The glass plate is well cleaned and desiccated, and then ex- posed to paraffin vapor by being kept in a horizontal position 5 cm above the surface of molten (not boiling) paraffin wax. Under favorable conditions hoar crystals very much like snow crystals can be obtained on the plate. The best example is seen in Fig. 5. The three stages of development of a hoar crystal thu
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