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CHEMISTRY 245 Antiseptic operations may be illustrated by a tank filled with sewage water, concentrated and rendered darker, if necessary, by evaporation. To this may be added a strong solution of potassic permanganate ; when the gradual clearing of the tank will be conspicuously shown. The difference between pure and ' hard' waters can be demonstrated by showing that the ' hard ' water, on adding certain reagents, gives precipitates which are not formed in distilled. If the water contains much lime, a crystal of oxalic acid may be tied to a thread and dipped into it, when distinct opaque threads of calcium oxalate will stream from the crystal, not apparent when distilled water is used. In all tank projections the same pains should be taken to get an evenly-illuminated disc, and by the same methods, as described in Chapter XIV. And any chemical apparatus or appliances may be readily projected, either by the focussing lens, or by the shadow method as described in that chapter. 132. Larger and Vertical Projections.—Some experiments and reactions are required to be shown upon a large scale, or on a more considerable flat surface of solution as a field. For such cases Professor Ferguson has given the arrangement in fig. 129, which enables vertical projections to be made with scarcely any addition besides the lantern itself, to apparatus which every chemical demonstrator has constantly at hand. The diagram almost explains itself. The lantern and radiant are so arranged that a somewhat diverging beam proceeds from the final lens, L, of the condenser. This is received on a mirror, M, adjustable at an angle of 45°, at the end of an arm sliding, and fixed by a screw, on the stem of a chemical retort-stand. Above it is arranged a large retort-ring holding a spherically-shaped glass dish ss, containing the fluid, and above is adjusted the small vessel containing the reagent, with a drop- ping-tube. The focussing lens—from the lantern itself, or otherwise—fits into a ring above at i, and a second smaller mirror in reflects the rays to the screen. The glass vessel s s