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 



