DURINGthe process of crystallization of many chemical salts, some unusually peculiar phenomena occur. It is well known that if a saturated or supersaturated solution of almost any salt is allowed to stand undisturbed for some time, many hours and sometimes days will elapse before crystallization will commence. While if the vessel is disturbed, or the liquid stirred crystallization will start immediately and the heat that was absorbed in producing liquifica-tion, will be given out during crystallization. A very good example of this was described in the issue of November 4th, under the heading “A Heat-Storing Water Bag.” When hyposulphite of soda is dissolved in water at ordinary temperature a considerable quantity of heat is absorbed as the following experiment will show. The temperature of the surrounding air was 70 deg. Fahr. Ten fluid ounces of water was taken and placed in a clean wide-mouth bottle, the temperature of the water being 60 deg. Fahr. Twelve ounces of granulated hyposulphite of soda was added to the water, and stirred with a glass rod until nearly dissolved. Upon inserting the thermometer the temperature fell from 60 to 30 deg. Fahr. and in the course of about one minute the temperature had fallen to 29 degrees. This experiment fully indicates the reason why in photography warm water must be used in the winter time in mixing the fixing solution, either for paper prints or plates, otherwise the time for fixing would be prolonged for a considerable period, and cause patchiness, in color, as well as an irregular fixed print. Another interesting experiment is as follows: Prepare a supersaturated solution of sulphate of soda Crystallization started with unsterilized glass rod. (this usually requires two ounces of the salt to one ounce of water). Heat two or three small glass tumblers and pour the supersaturated solution into each. Cover the top of each vessel with a piece of sheet glass, and let the tumblers stand undisturbed until they have become quite cold. It is well to prepare the vessels at night, allowing them to stand undisturbed until the morning. They will then be ready for the next step in the experiment. Take a glass rod, about 9 or 10 inches long and a quarter inch diameter. Make one end red hot for about two inches, in either a spirit lamp flame, or a Bunsen burner. Suspend it to cool, and then stick a piece of gummed paper about five inches from the end that was heated, in order to-identify it. As soon as the rod is quite cold, remove one of the glass plates from the vessel containing the cold solution of sulphate of soda, insert the end that has been heated, and stir the liquid. No crystallization will take place. Turn the rod around, insert the end that has not been heated and stir the liquid. Crystallization will begin at once, and will continue until the liquified salt has become a solid mass. The experiment can be repeated with the second or third vessel with the same result, providing the salt has not crystallized upon the end that was originally made red hot. If crystallization has taken place, the end must be well washed by pouring warm water over it. This peculiar property has been known in some instances to be retained by the glass rod for two weeks, when not touched, and kept from the atmosphere. The experiment is not a costly one. Hyposulphite of soda can be purchased at almost any drug store for five cents a pound, and sulphate of soda at about the same price. The illustration in Fig. 1 shows the stirring without causing crystallizing; Fig. 2 shows the growth of crystals from the rod in the center of the saline solution.
This article was originally published with the title "The Laboratory" in Scientific American 105, 25, 560 (December 1911)