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its surface. The more volatile ingredients of the balsam are dispelled by the heat, and when that is accomplished the smooth, dry, warm rock is pressed firmly into contact with the glass plate so that the film of balsam intervening may be as thin as possible and free from air bubbles. The preparation is allowed to cool, and the rock chip is again ground down as before, first with carborundum and, when it becomes transparent, with fine emery until the desired thickness is obtained. It is then cleaned, again heated with an additional small amount

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