Lecture Demonstrations with a Shadow Screen The Vapor Pressure of a Crystal Hydrate E. K. BACON Union College, Schenectady, New York
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ECTURE demonstrations are frequently ineffective viewed from the front. With a darkened room the efbecause the demonstration is not clearly visible to fect is striking. Introduction of a liquid of high vapor the audience. The poor visual character of a lecture pressure such as ether is accomplished by means of a table experiment is illustrated by the well-known dem- bent medicine dropper. Figure 1 shows a photograph onstration of the vapor pressure of a liquid. Here a of the demonstration taken a t a distance of about four liquid of high vapor pressure introduced into an in- -feet. The clarity is as satisfactory when viewed'from verted tube filled with mercury causes a drop in the the rear of a large lecture room. It is interesting to note height of the mercury column. Light rdections and the ready visibility of the liquid resting on top of the the small size of the tube make the result visible only to mercury column. those nearest the lecture table. Another experiment that works equally well with the An improvement in the effectiveness of this experi- shadow screen is the demonstration of the vapor presment resulted when only the shadow of the apparatus sure of a solid in the form of a crystal hydrate. A dry was exhibited in a darkened room. This was ac- 1.5-liter flask is fitted with a two-hole rubber stopper. complished by the construction of a plain wooden frame A short piece of 6-mm. glass tubing is inserted into one with inner dimensions of approximately 18 X 24 hole and to this is fastened a piece of rubber tubing inches. Tracing paper was stretched tightly across the with a screw clamp. Into the other hole is inserted back of the frame and held in place by means of thumb another piece of 6-mm. tubing bent suitably in the form tacks. A piece of ply wood tacked to the side served as of a manometer. Kerosene colored with an oil-soluble a support. dye may be used as the manometer liquid. With With this simple shadow screen the vapor pressure the screw clamp open crystals of NalCOa.lOHaO or demonstration may be carried out as follows. -The glass NasSOc 10HzOare intioduced into the flask, the stopper tube is filled with mercury and inverted over mercury inserted firmly, and the screw clamp closed. Within in a beaker on a small stand. The tube is held by an the period of several minutes the liquid rises slowly and apparatus clamp and support. The screen is placed in reaches a steady state. The difference in height of the front of the tube so that direct contact is made with liquid in the arms of the manometer is of the order of 30 paper. A 100-watt lamp placed a t an appropriate dis- mm. tance back of the screen gives a clear shadow when When viewed in the shadow screen the result is very
effective. The refraction of light by the oil gives a brilliantly illuminated colored column that is clearly visible a t a considerable distance. The photograph of Figure 2 gives an accurate view of the clearness with which the experiment may be observed. Details of the construction of the apparatus can be noted. The bend in the tube above the column of liquid is a t an angle
perpendicular to the plane of the screen and the U of the manometer. Only the U of the manometer touches the screen. No doubt many other lecture demonstrations may be shown by the shadow screen with good results. The writer would be interested to hear from others who may wish to experiment with it.
