LEAD TREES GROWN IN GELS G . P. BREWINGTON.HIGHSCAOOL, GOLDEN, COLORADO'
It is a well-known fact that if a zinc or tin rod is placed in a lead nitrate solution, long filaments or trees of lead will grow out from the metal. The trees so formed are very fragile and must be handled very carefully and even then are apt to break. If such reactions he allowed to take place in a gel the branches are much less liable to break as they are supported by the gel. Since the gel will probably be new to more readers than that of the lead tree i t will be discussed briefly. The following has been found to work very well in their preparation. Two stock solutions are necessary. The first a normal solution of acetic acid. This may be approximated if 60 cc. of glacial acetic acid is diluted with water to make one liter. The second is prepared by diluting sodium silicate (water glass) with water until the mixture has a density of 1.06. Commercially available water glass is apt to vary within rather wide limits so no approximate volumes can be given. If equal volumes of the two solutions as prepared are mixed and set aside they will congeal or set into a milky translucent solid known as a silica gel. The time required for setting will depend upon the concentrations of the two solutions and the presence of certain dissolved salts. Owing to variation in water glass i t would he well to see that the mixture is acid to litmus paper and if not add more acid solution accordingly. It has been found that the gels requiring a longer period of time to set +re the more desirable to use in growing trees. A few words regarding the properties of silica gels might be in order. The gel does not form instantaneously but seems to progress gradually, as the solution becomes more and more translucent and viscous until the whole mass becomes solid. Gels prepared with more concentrated solutions than those described become opaque, hut many made as described and requiring long periods of time to set were very clear. Should the container be about the size of a pint milk bottle the gel will be found to have the ability to vibrate. This is easily demonstrated by holding it firmly in one hand and giving it a sharp tap with the other. By filling a series of containers with varying amounts of solution and permitting i t to set, a musical scale might he arranged so that by tapping the container with a pencil a tone will be emitted-hence a colloid xylophone. Some care must be exercised or the gel will fracture in rough handling. The fracture, it will be noted, is of the conchoidal variety somewhat similar to that of glass. The lead trees may he prepared by suspending or placing on the bottom of a glass container several pieces of tin, adding equal volumes of the two Presmt address, University of Michigan, Ann Arbor, Michigan.
solutions as prepared and put aside until the whole mass is set. This may take twenty-four hours. Then prepare a saturated solution of lead nitrate and place this on the surface of the gel. This accomplishes two things, it prevents the gel from drying out and subsequently cracking and supplies the lead ions which will now diffuse into the gel and be displaced by the tin. The reaction may start almost immediately or sometime after-in one case almost a month elapsed. Should the experimenter desire he mav dissolve the lead nitrate in the acetic acid solution before
mixing with the silicate solution. The lead nitrate will accelerate the process. Almost every tree grown is different. Generally the branches are twisted and have oddly shaped leaves. I have seen one in which a branch on reaching the glass wall is reflected such that the angle of "reflection" approximates the angle of "incidence." Another interesting tree is formed if two clean lead electrodes are placed I am indebted to Dr. R. E. Nys2 These gels were prepared by Keith Wilhelm. wander for the use af special photo~aphicequipment.
in the container before the solution, containing the lead nitrate in this case, has had time to set and a weak current sent from one electrode to the other. It is well to place a 10-watt light in series with the circuit to prevent large currents from flowing as they are very apt to cause the gel to tear away from the electrodes. A large number of the gels and trees have been made by the science club and chemistry classes of the high school a t Golden, Colorado. The growing of these trees is recommended as an instructive but inexpensive project for science classes and clubs. For further discussion the "Laboratory Manual of Colloid Chemistry" (second edition) by Harry Holmes is especially good. Holmes generally recommends more concentrated stock solutions than are given here but we have had few failures to set.
