(44) A. G. asks: Is the silver, for a reflecting telescope, put on the back of the glass the same as on looking glasses? A. No. Only one side of the glass is ground and polished to the shape required. The silvering is done on this side; and then, with the softest buckskin and the finest rouge, the surface of the silver is polished for the reflecting surface. In cities where gas is used, it will not retain its brilliancy very long; then it requires to be cleaned with nitric acid and resilvered, which is only the work of a few hours when a person has become accustomed to it. (45) A. L. B. says: 1. I understand that, in modern chemistry, the acids and alkalies are the two extremes of a class of substances called hydrates, the only difference being the radical. In the reaction of nitric acid, HO NO_{2} or HNO_{3} on potassic hydrate, KOH is KO NO_{2} or KNO_{3}, and H_{2}O. Which molecule loses the oxygen atom, and why should one part with it more than the other? A. HNO_{3} + KOH = KNO_{3} + H_{2}O Nitric acid potassic potassic water hydrate nitrate In this reaction the potassium is considered, by virtue of its greater affinity, as replacing the hydrogen atom in the hydric nitrate; the hydrogen in turn satisfying the OH group to form water. These hydrates are similar only in point of constitution. Their chemical deportments are widely different. 2. What are oxides in modern chemistry? A. The bodies formed by the direct combination of oxygen with the elementary bodies are called oxides. With water some of these oxides form hydrates, as K_{2}O + H_{2}O = 2(KHO) potassium water potassic oxide hydrate (46) J. R. M. asks: To have a circular saw run well, should the mandrel have a little end play if it is desired to relieve the saw and guides of strain? A. If the saw is not true or the carriage runs crooked, end play of the mandrel to the extent of the deviations will relieve the strain upon the saw. But if the carriage runs true and the saw true, the mandrel should have no end play. MINERALS, etc.--Specimens have been received from the following correspondents, and examined, with the result stated: M. S. M.--It is a quartz crystal, the opposite sides of which have been ground flat, probably by artificial means.--F. B.--It is graphite. * * * * * COMMUNICATIONS RECEIVED. The Editor of the SCIENTIFIC AMERICAN acknowledges, with much pleasure, the receipt of original papers and contributions upon the following subjects: On a Battery and Electric Clock. By J. E. W. On Anti-Water Drinking. By C. P. W. On Snakes Catching Fish. By C. R. G. On Utilization of Sewage. By Dr. H. D. T. On Aerial Navigation. By C. W. On the Ash-Colored Salamander. By C. F. S. * * * * * HINTS TO CORRESPONDENTS.