WEATHER PROGNOSTICATORS. 131 



Expt. 131. Chemical Weather-Prognosticator. Strips of paper 

 impregnated with similar fluids are sometimes used as" weather- 

 prognosticators. If the air be comparatively free from moisture, 

 and a long way from being saturated with aqueous vapour (Expt. 45), 

 it tends to exert a drying action on the cobalt salts, causing a 

 lilac or blue shade to become visible ; whilst if the air be nearly 

 saturated with moisture, the blue disappears, and a pale pink only 

 is seen; so that violet and blue colours indicate that fine weather 

 is probable, at any rate for awhile, and pink tints indicate that 

 the atmosphere is more or less waterlogged, a condition of things 

 probably tending towards rain. A strip of sea-weed dried in the 

 sun is impregnated with the solid constituents of sea-water, and 

 if suitably chosen can be similarly Used as a weather-indicator ; in 

 damp air the saline matter present takes up moisture, and makes 

 the weed limp and soft ; whilst in fine weather it dries up again, 

 and becomes more or less hard and rigid. Catgut, hairs, and other 

 fibres of animal origin are somewhat similarly affected by moisture. 

 A toy weather-indicator often met with consists of a strip of wood, 

 &c., balanced nearly horizontally on a pivot but not quite, so that 

 the heavier end has a tendency to move forward; at the two 

 ends of the lever are placed figures of a man and woman respec- 

 tively, and a catgut thread is attached to the lever in such a way 

 that when the thread shrinks it draws in one side of the lever 

 which is placed in the entrance of a miniature doll's house, so 

 that one figure comes out and the other goes in ; the arrangement 

 being usually such that the man comes out in wet weather and 

 the lady, gaily attired, in fine. 



Expt. 132. To precipitate one Solid whilst dissolving another. 

 T Put into a wine glass a little solution of chloride of copper, and 

 then dip into the liquid a clean iron key or bright knife-blade. 

 In a few seconds you will find that a film of red metallic copper 

 has formed over that part of the metal immersed in the solution. 

 This is due to the circumstance that the chemical change called 

 replacement or displacement (or " single decomposition ") takes 

 place (Expt. 9). The original fluid was a solution of a substance 

 containing two constituents, copper and chlorine respectively ; when 

 the iron key comes in contact with this, the iron " decomposes " 

 the compound of copper and chlorine in such a fashion that copper 

 is set free in visible form, whilst the iron combines with the 

 chlorine, forming a new compound, chloride of iron. This 

 latter substance is invisible to the eye, as it dissolves in the water 

 as fast as formed; but its presence may be shown by chemical 

 tests, thus Let the key stop in the solution for an hour or two ; 

 by this time a pretty thick deposit of powdery copper will be 



