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JOURNAL OF CHEMICAL EDUCATION
caustic soda solution, uzhich no doubt helps to reduce the solubility of iodoform. The above method may further be used for the quantitative estimations of both alcohol and acetone, in very dilute solutions, by utilizing centrifuge tubes (calibrated) of suitable dimensions.
drops of a 5% solution of Na2S.9H20. This solution may be kept on hand in a glass-stoppered bottle. It must be renewed from time to time. W. JACOBSON CHICAGO, ILLINOIS
I. BISWAS T. N. J. COLLEGE BHAGALPUR, BEAAR(INDIA)
T o The Editor: I was very interested to see the recentarticle which appeared on page 342 of your journal dealing with. methods of detecting carbon monoxide. I think you willperhaps be interested.. .in a somewhat analogous method which has been developed in this country and which we are now marketing here. Whereas the silica gel molybdate tubes have to be matched against the color chart, the "CO-test" tubes which we are selling produce a brown stain the length of which can be measured by a pocket ruler. In order to be able to draw a sample through tubesfor fixed time a t a constant rate, a double aspirator is used which can be reversed when all,the water has flowed from one into the other. hi^ makes a very simple compact arrangement which is becoming increasingly popular in chemical factories, gas works, and wherever danger of co poisoning has to be guarded against.
L. T.
MINcHIN
COMBUSTION INBTRUMENTB 106 K E N S I N ~HIGH N STREET,LONDON, W. 8
To the Editor: EDUThe March issue of the JOURNAL OF CHEMICAL CATION features an article by Gaines and Woodriff (page 166), describing the separation of Cu from Cd without the use of KCN. I am in fyll agreement with the authors that the use of KCN should be avoided in freshman laboratories. Having had myself some disagreeable experiences with H& I personally am just as much afraid of H2S as I am of KCN. The use of H2S cannot be avoided in qualitative analysis, all schemes for analysis with other reageilts do not work as smoothly, but it seems to me that in the procedure under discussion the application of H2Sis not necessary. I would suggest that the separation of Cu from Cd be done in the way outlined by Gaines and Woodriff, but the testing for Cd should not be done by bubbling H2S through the final filtrate, but by adding to it a few
T o the Editor: I was glad to have the opportunity to look over the article submitted by Mr. Seifert on "A Rapid Method for Adjusting Grades to a Standard Scale." It is a good statistical analysis of some of the principles underlying grade adjusting and it goes beyond my article (Tars JOURNAL, 25, 690 (1948)) in that it offers a different scheme which, under ideal conditions, would be superior to mine. m e t h e r it is superior under ordinary conditions remains for the teacher to decide after he has examined the two methods. Perhaps You recall that I too suggested that a slide rule might be used, but said that it was only useful in adjusting those grades that fell below the average of the class. This is equivalent to using Seifert's equation (1) in which the raw score is multiplied by a constant factor MdM. Seifert's scheme actually employs equation (2) for all scores, above and below the average. There are a number of drawbacks to Seifert's scheme: (1) whenever the highest grade on an examination or other exercise is not 100 or very near to it, it becomes necessary to multiply each grade by a factor which will make the highest grade 100. This operation takes as much time as the subsequent adjustment itself. (2) When equation (2) is used (and the special slide rule) the assumption is made that the average raw score lies halfway .between the lowest and the highest score. Seifert says that in most cases the distribution of grades is a sufficiently close approximation to the normal curve to warrant making this assumption. This may be so in general but it has not held with a sufficient degree of approximation a t New York 'University where the actual grade distribution curve has sometimes been rather much askew. (3) If the "standard" average is assumed as 75%, a student with a raw score of Ois boosted to 50. All very low scores receive unusually high increments. This gives a student who is used to the C-100% ratings unwarranted confidence and does not encourage him to multiply his efforts; it may even mislead him into thmking that somehow or other the system is giving him something for nothing. WILLIAMF. EHRET NEWYORKUNIVERSITP W M ~ N O T OSQUARE, N NEWYORE
