VOL. 6, No. 4
W
~
IsT TRUTH?
i17
chloride molecules which are precipitated. True, this is in terms of the electrolytic dissociation theory but theories are for the purpose of generalization and the following facts cannot be brought under the canmon ion principle: saturated solutions of sodium chloride, potassium iodide, and potassium bromide are each precipitated by hydrogen chloride, hydrogen bromide, and hydrogen iodide and also by ammonia gas. I think that Mendeleell would generalize and say that these salts are not as soluble in the strong solutions of the gases as they are in pure water, and would be not thus miliciently explain the situation? In the cases where there is a common ion (He1 NaCl, HBr KBr, and HI KI) doubtless the canmon ion principle should be considered but when there is no common ion and we get precipitation, it is evident that our teaching must include some other factors. The three hydrogen halides are very soluble in water and cause precipitation of the salts. Ammonia gas is also very soluble in water and it, too, precipitates salts from saturated solutions of sodium chloride, potassium bromide, and potassium iodide. A number of interesting questions come up a t this point but my purpose is not to explain how these precipitations take place nor even to tell what these precipitates are. I have not stated that hydriodic acid precipitates sodium chloride nor am I sure that this acid precipitates only sodium chloride. I did state that there were precipitations. I am,however, questioning the advisability of teaching a series of phenomena in terms of a theory which cannot hold for all cases. In other words, until we know what is truth, should we state the facts as they appear and offer explanations which are not in accord with the facts? Is the common ion effect a teachable explanation for very strong solutions of salts which are precipitated by the addition of very soluble gases? A teacher dislikes to call the attention of his classes to the shortcomings of the textbook. Misstatements and especially wrong and impossible explanations when brought to the attention of the pupils make them lose confidence in the text and perhaps in the teacher, too, or lead them to think that it is more or less guesswork and that "one man's guess is as good as another's." This should be avoided, if possible, and in order to eliminate some of these, perhaps minor, defects in our textbooks, this paper has been presented. After Pilate asked the question, "What is truth?" he said, "I fmd no fault." I, too, would prefer to be classed with the truth-seekers rather than with the fault finders.
+
+
+
British Output of Rayon. The British rayan industry is now producing at the rate of, roughly, 51,250,000 lb. a year, compared with 25,500,000 lb. in 1926. During 1928 there was a tendency for exports to increase and far imports to decrease, imports being now about 2 million ib. per annum compared with over 10 d l i o n lb. in 1924.Chem. & Ind., 48, 145 (Feb. 8, 1929).
