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Most chemists now assume that the various crystalline modifications of the pentose and hexose sugars do not possess a free carbonyl group in the molecule but have a 5-atomic lactone of the general typeFrom this standpoint a and ß d-glucose have the same lactone and differ only in the space relations about the end asymmetric carbon atom which in one form has the structure in the other, the structure This same relation is supposed to hold for the a and ß forms of all pentoses and hexoses as well as for the corresponding glucosides.A careful comparison of the configurations of the sugars with their optical rotations led the author some years ago to conclude that the above-described assumptions may be incorrect and that a h-glucose may have a 4-atomic lactone while ß h-glucose may have a 3-atomic lactone,2 and, further-1 Contribution from the Massachusetts Agricultural College.