Correspondence
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EQUALITY AND SIMILARITY IN CHEMISTRY We characterize any object by the sum of its properties. Though two (or more) objects are never entirely alike, we nevertheless look upon them as being alike even if they can only be said to be "similar." Two chicken eggs, two leaves of the same plant, two crystals of snow, are to all appearances similar, but never alike in every detail. This seems to be a very obvious statement, i t may even be termed trite. However, we chemists should keep this fact very much in mind, when we claim equality for two or more substances from a chemical point of view. I venture to ask the question: Did any one of us ever stand before, or .possess, two beakersful of distilled water of which he was sure that the two equal quantities were really alike in every detail? The answer cannot be but doubtful. And, if this be true for water, i t will likewise hold for any other substance which may be chosen for a comparison. We do not go as far as all that. We are satisfied that the water in each case will show the same freezing point, and, under the same conditions of experiment, the same boiling point. It will a t
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definite temperatures dissolve equal amounts of water-soluble material, and so forth. It will, above all, when subjected to electrolysis, furnish hydrogen and oxygen in definite volumes; it will give us hydrogen when treated with the alkali metals. All of which satisfies us that we have in both cases the same substancewater-before us. A reasonable sum of properties suffices for our proof of statement of equality. And we let i t go at that. We state that when we find this sum of properties, we have to do with water pure and simple. But, nevertheless, we must not forget that we have actually only stated a case of similarity. One more case of presumable superfluity of evidence is to be mentioned. Has any one ever in a case of water of crystallization really identified such water? The alums would furnish abundant chance for such an example of neglected research. After all, we may state that, though chemically pure water is a substance rarely, if ever, met with, we are fairly sure in identifying water by its properties. We have intentionally taken water as a substance to start with, due to the simplicity of the case. What we have considered so far may be termed a preamble. We are driving at something more complex. And, in order to gradually arrive a t that, we go back to those objects which we term elements. Let us, for argument's sake, define an element as a substance which will show a definite spectrum, such as will not be shown by any other object. Also here, we find from the start that the ideal case will seldom, if ever, materialize. Let us neglect the minute quantities of pollution. We know that chemical purity does not exist. But a grave problem arises outside of that. It is the fact of what we term allotropy. We claim for sugar-carbon, for graphite, and for diamond the elementary state. We claim that all three are carbon. Imagine a non-chemist before whom you spread in three different portions, say, lampblack, graphite, and a perfect diamond, and tell him with the most serious face of the world: now these three objects are all one and the same thing. Would not our friend, the non-chemist, look to yon as if a t one slightly demented? Would be not ask: how do you make that out? And if you told him that by some sort of complex reasoning you can prove to him that all three of these substances will yield upon combustion the same carbon dioxide, he would be by no means wiser. Suppose they do, he might reply, does that belie the fact which I perceive with my eyes, that these three substances are as different from each other as night and day? You would then tell him that carbon dioxide must of necessity be produced from something that is or contains carbon, since we cannot transmute the elements in such a manner as to obtain carbon dioxide from any other element but carbon. And, that since these three are in their entirety transformed into carbon dioxide, they must all three he exclusively carbon. Though, you add, we see from these facts that the element carbon can exist in three different
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JOURNAL OP CHEMICAL EDUCATION
OCTOBER, 1928
forms, or conditions, since the appearance and the readily recognizable properties of the three differ widely. That is a bit of chemical reasoning which your friend may or may not understand, but which is valid for us chemists. You may further add that graphite as well as diamond can be prepared from the ordinary carbon, either, as for graphite, in the electric furnace or, for diamond, from the solution of carbon in fused iron. For phosphorus the case would be just as phenomenal, considering the difference of the yellow and the red variety in behavior. Here, however, the transformation of one form into the other is more readily demonstrated. Let us assume (as is probably the case) that for all elements there exist such allotropic forms, where do our terms equality or similarity come in? We must assume such substances as similar (or equal) which, when brought together with a third substance, will produce the same fourth body. Thus, in this case one fact, and this one fact only, will prove similarity or equality. And, finally, we approach those cases which are so abundantly found in organic chemistry and for which we have the term isomerism. Two (or more) substances which are different in all their properties, except in the one that they will yield upon combustion, say, carbon dioxide and water, to the same amount; both are considered as produced from the same ultimate elements. Also here, the one fact of production of a fourth (and fifth) body counteracts all of the other observed facts of dissimilarity and makes the two isomers in a sense equal. And, in case of the existence of hundreds of isomers, all of them, in a sense, would have t o be considered as equal or similar. Equal or similar in what? I n one case of behavior only. Why is it that this one case of behavior overshadows all of the other cases of dissimilarity? As we stand here before a multiplicity for which w r ignore the real cause, we have attempted to hypothesize a cause, but let that not lead us astray; let us rather state that we have no measure for equality and similarity in chemistry, that we are far from understanding the scope of possible multiplicity in chemical interaction. Percentage of elementary matter which we can get out of a given substance in the form of a new substance, will not tell us anything clear and new about our great variety of isomers. L. H. FRIEDBURG 601 W. 148Tn STREET, NEWYORKCITY
One oi our correspondentsdesires information relative to John Hamson (1773-18331, one of the earliest manufacturing chemists in the United States. Any suggestions will be appreciated.
