To the Editor: There have come to my attention the following three independent reports on polyethylene infrared cells (J. CHEM.EDUC.,39, 262 (1962)). My apologies to these authors for not crediting their work: ROBINSON, T., Nature, 184, 448 (1959); FABBRI,G., Ann. Chim. (Rome), 50, 199 (1960), in CA, 54: 19164b (1960); and ARREGUIN, B., Bol. Inst. Quim. (Mexico), 12, 51 (1960). Robinson suggested heat-sealed polyethylene envelopes for aqueous solutions and some other liquids whereas Arreguin's suggestions were more similar to those made here. I believe that this not only well illustrates the Zeitgeist principle but also demonstrates the effectiveness in communication of these pages, which brought this to light.
To the Editor: In an article in THIS JOURNAL, 38, 570 (1961), James P. Hoare described a method for determining the slope a t any point on a curve, employing a round glass rod placed across the curve a t the point. I n a subsequent article in THIS JOURNAL, 39, 299 (1962), C. J. Hochanadel described a device to overcome the difficulty of holding the rod in place while drawing the line. It seems appropriate to direct attention to a clever device designated a "tangentimeter" and described by Latshaw [J.Amer. Chem. Soc. 47, 793 (1925)l. This device consists of a steel square with a mirror mounted on one leg so that the reflecting surface is normal to the edge of the square. When the mirror stands across a curve and is normal to it, there is no break between the curve and its image. A line drawn along the side of the square has the slope of the curve a t the point where the mirror stands. This device is referred to in Livingston's "Physicochemical Experiments," 3rd edition, Macmillan Company. 1957, p. 43, and in both editions of the classic manual by Mack and France, "A Laboratory Manual of Elementary Physical Chemistry," D. Van Nostrand Co.; 1st edition, 1928, p. 51: 2nd edition, 1934, p. 81, now out of print. There is available commercially from Edmund Scientific Co., Barrington, N. J., a device (Cat. No. 50226, list price $3.00) which is a refinement of the Latshaw device. It consists of an optically flat piece of halfsilvered glass. When it sits across a cuwe and is normal to it, the reflected image of the portion of the curve lying in front of the device coincides with the transmitted image of that portion of the curve lying behind the device. With the device in this position, a line drawn along its front edge is normal to the curve. To construct a tangent to the curve it is necessary to use the device a second time, placing it so that i t is perpendicular to the normal.
To the Editor: While working with silver nitrate, we rediscovered the problem, common to many chemistry students, of removing silver stains from skin and clothing. A method which we found to he both simple and effective is rubbing the stain iirst with a solution of KI and I2 in aqueous ethanol. A wide range of concentrations seem to be effective; commercial tincture of iodine may be used. When the stain turns yellow, it should be rubbed with a saturated sodium thiosulfate solution and then rinsed with water. All stains should be treated as soon as possible; and in removing stains from clothing, make sure the fabric is not attacked by iodine. GERALD J. TENNENHOUSE
To the Editor: Since the completion of the manuscript for Dr. Wise's article (see THIS JOURNAL, 39, A 771 (1962)-October), the Kaman Model ST-60-7 and KT-60-8 Pulsatrons have been supplemented by the Mark I V which also uses a sealed tube but embodies principles which increase the life to 2.5 X lo5 pulses. The price of the Mark IV is $8500. The exchanee of the accelerator tube is - vrice . $500. A second type accelerator now available is the Mark V. This unit uses a 140 kv power supply to accelerate ions onto a tritium target, providing an output of loa neutrons per second. The ion source is a PIG type, the gas being supplied by an electrically heated zirconium wire. The target is cooled and is at ground potential to permit the placement of samples within 0.1 in. of a target surface. The complete unit is priced at $9500. The exchange price for the accelerator tube is $500. The Kaman differentially pumped accelerator source has been expanded to include a line of three basic units. The Model NT-60-9 which is now priced at $12,500 is supplied 100 kv to produce a beam of 500 microamperes and an output of $5 X lo9 n-sec-'. The recently developed XT-60-10 and ST-60-11 have outputs of 3 X 101°and 10" n-sec-', respectively. The NT-60-10 unit operates at 130 kv and 1 ma beam. This unit is priced at $18,000. I t is also available with a 5 X 10'O n-sec-' output from a 160 kv power supply for an additional $1000. The NT-60-11 unit operates a t 200 kv with a 1 ma beam and sells for $22,000. ill1 prices quoted include the control console and power supply. These differentially pumped units use a PIG source to provide simplicity, ruggedness, and long life. Also, in the -10 and -11 units, the target life has been increased by providing a mass analyzer in the drift tube so the monatomic component of the beam may be used alone and by providing a means of replenishing the target within the tube. The target can be replenished whenever the yield decreases, enabling the neut,ron generator to oper~tecontinually at near maximum output without the necessity of target replacement. KENNETH W. ERIKSON
Volume 39, Number 1 1, November 1962
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