the spectral discharge tubes is that our lecture halls have sloped floors, so the heads of the students in the rear are about 15 ft higher than those in the front. The grating's curve and length allow viewing of the spectra throughout a horizontal arc of about 140°, adequate to range from one front corner of the lecture hall to the other. Different classroom geometries will dictate variations in the size and shape of the grating frame; I have described one design that best fits our lecture halls. Our setup easily displays a continuous spectrum from an incandescent or fluorescent lamp. If the room is quite dark, it also makes visible the superimposed bright-line spectrum due to the mercury inside the fluorescent tube. The argonplus-mercury source shows three bright lines of the discontinuous mercury spectrum, one orange-yellow, one green, and one blue-violet, corresponding to the triplet about 5790 nm, the single lineat 5460 nm, and the single line at4358nm. The neon source displays a series of closely spaced lines ranging fromred through orange and yellow to green. A faint green-blue line is also visible. Acknowledgment I wish to acknowledge Ronald Perkins and James Weisshaar for their valuable contributions in preparing this manuscript.
The Endothermic Dissolution ot Ammonium Nitrate SUBMITTED BY
George 8. Kauffman and Craig A. Ferguson Calltornla State Unlverrltg, Fresno Fresno, CA 93740 CHECKED sv
Arnold George Manslleld University Manstleld. PA 16933 The dissolving in water of substances that are more soluble a t higher temperatures (the majority of salts) is an endothermic p r o c e ~ s . lThis ~ ~ lowering of temperature can be shown i n a strikine manner hv the followina demonstration. About 5 drops i f water are placed on the center of a dry mahogany block (we used a block -13 X 9 X 9 cm, weighing 620 g) that has been fitted with nylon fishing line (at least 12-lh test) so that weights can later be suspended vertically below it. A 150-mL wide-bottom beaker (Pyrex no. 1000, thin elass, if ~ossible)with a perfectly flat bottom containing :I; mL ofwarcr that has previously and surreptitiously heen cooled to 0 "C in an ice bath (-15-20 minl'is placed in the exact center of the puddle formed, and 14 g of ammonium nitrate is immediately added nll or once with vigorous stirring, while holding down the heaker firmly on the bluck withone hnnd.' Withina minute the beaker u,ill be fruzen to the 11lockso tiehtlv that the block mil\. be lifted .ilou.ly and uertically from thk lecture table by ;sing the beaker as a handle. If weights are addedgradually and care is taken that the heaker and weights are perpendicular to the floor, as much as 18 kg (-40lh) (including the block) can he suspended from the heaker if the weights are added within 5 min (see
medemonstration (see text).
figure). Goggles should he worn, and the beaker, which should be stoppered with a no. 12 rubber stopper or cork to prevent splashing, should be held with a towel or rubber glove in case the beaker becomes detached from the block. The NHaN03 may he recovered by crystallization. SafetylDlsposaI . . Although ammonium nitrate is a strung oxidiling agent in the dr,v state and can initiate romlu~rionin flammahle matrriah, it is very solublr in water, and its aquems strlutions are not dangerous. The solution may 11ediscarded bv flushing down the drain with water
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Acknowledament Theauthori wish toacknowledge the assistancerd hlanud Da Costa oithe CSI.'F Department of Physics. Alvea. N.: Dunon. F. B.. Eds. TestedDemonstrationsin Chem , ~H. ~ . istry Journal of chemical ~oucation:Easton. PA. 1965;p 17. Sna
