236 



THE POPULAK EDUCATOR. 



will become milky, owing to the precipitation of the salts. A 

 similar effect may be produced by adding a little lime-water ; the 

 lime appropriates the carbonic acid, and consequently the salts 

 originally in the water and the newly-made carbonate of lime 

 are precipitated together. 



The formation of stalagmites and stalactites in caverns, and 

 likewise the petrifaction of bodies placed in the water of many 

 wells for example, the " Dropping Well," at Knaresborough, in 

 Yorkshire is due to this suspension of calcium carbonate by 

 carbonic acid gas. When the water is distributed over the sur- 

 face of the body undergoing petrifaction, or as it drips from the 

 roof of the cavern, the carbonic acid gas escapes, and the calcium 

 carbonate, being no longer capable of existing in solution, is 

 deposited. 



When water is required for delicate experiments, for pho- 

 tographic purposes, or for making solutions for laboratory use, 

 distilled water must be used. 



To distil water, the glass tube in Liebig's condenser should be 

 replaced by a metal one lined with block tin ; and to be quite 

 sure that the water in the retort is not acid, it is safer to add 

 a little carbonate of soda. Acidulated water, after the process 

 of distillation has been continued for some time, begins to be- 

 come acid. 



Water should never be kept in a lead cistern ; for if it con- 

 tain any carbonic acid gas, a lead carbonate would be formed, 

 which renders the water poisonous. 



Water is a perfectly neutral body, being capable of combining 

 with acids and likewise with alkalies. When it enters into 

 chemical combination with a substance it forms a hy 'drate, while 

 a body which contains no water is said to be anhydrous. 

 Crystals also contain water, which is necessary to their exist- 

 ence; it is called the "water of crystallisation," and when 

 driven off by heat the crystal falls to powder. It is thus ex- 

 pressed in symbols 



Na,O,CO, + 10 HO 4 = soda crystals; 



that is, sodium carbonate and ten atoms of water of crystallisa- 

 tion. 



Water is a great solvent ; being capable of holding in solution, 

 more or less, almost all bodies Asa general rule, more of the 

 body is dissolved by hot than by cold water. When a quantity 

 of water will not take up any more of a substance, the solution 

 is said to be " saturated." If it be warm water, then, on cooling, 

 the dissolved substance will be deposited in crystals ; if cold, the 

 crystals will appear on evaporating. This may be interestingly 

 shown by adding alum or salt to boiling water until the water 

 will dissolve no more; then suspend a bunch of thread, etc., in 

 the vessel, and set it to cool in a dark, quiet place. The greater 

 the quantity of the solution the larger will be the crystals on 

 the thread. 



The ocean is the great reservoir of water. But sea-water 

 holds in solution a great number of salts. The effect of this is to 

 increase its density ; hence ships can bear much greater cargoes 

 in salt than in fresh water. As we shall find that some elements 

 are always got from sea-water, we give an analysis of that in 

 the British Channel : 



Water 



Sodium Chloride 

 Potassium Chloride 

 Magnesium Chloride 

 Magnesium Bromide 

 Magnesium Sulphate 

 Calcium Sulphate 

 Calcium Carbonate 

 Iodine .. 

 Ammonia .. 



963-74372 

 28-05948 

 076552 

 3-66658 

 0-02929 

 2-29578 

 1-40662 

 0-03301 

 traces 

 traces 



1000-00000 



Ships are now fitted with apparatus for distilling sea- water ; 

 but the fresh water thus procured is insipid ; if, however, ii 

 be filtered through charcoal it is " aerated," and assumes the 

 refreshing taste of spring water. 



All water contains air ; but the air in water has double the 

 proportion of oxygen in it than in atmospheric air. This is one 

 of the chief uses of aquatic plants they give off oxygen to the 

 water. Fish, which require oxygen for their existence, pass the 

 water through their gills, which apparatus retains the gas. If 

 oxygen were in the same proportion in the air in the water as in 



ihe atmosphere we breathe, the ttsh would nave to pass through 

 ;heir gills double the quantity of water thus entailing much 

 greater wear and tear of life. The fact that fish require air may 

 je shown by boiling water, which expels the air from it, allowing 

 t to cool, and then putting some " gold and silver " fish into it 

 The animals will come to the surface to breathe, and endeavour 

 jo " aerate " the water by agitating it with their tails. 



The last form we shall notice in which water is given to the 

 earth, is dew. 



During the day, as we have seen, the sun evaporates mois- 

 ;ure, filling the air with water-vapour steam. When the sun 

 sets all bodies begin to radiate off their heat into the sky. The 

 leaves of the vegetation are good radiators of heat, and there- 

 fore the grasses, etc., soon lose their heat and become cold. But 

 not only are they good radiators, but they are also bad con- 

 ductors, and therefore they receive no further addition of heat 

 from the earth out of which they grow ; upon their cold surfaces 

 the moisture in the air is condensed, forming dew. 



No dew is found on the bare earth, because it is never cold, 

 beat being radiated from it all night ; neither do we find dew 

 under the overspreading branches of trees, nor on a cloudy 

 night; for the branches and the clouds throw back the heat 

 radiated from the earth, and thus the grasses beneath them are 

 never cold. On a windy night the vegetation is warmed by 

 " convection " that is, the disturbance of the atmosphere 

 brings in contact with the blades of grass, etc., warm particles 

 of air, and by this means their temperature is kept up. 



Thus, the conditions for a copious fall of dew are a hot day, a 

 clear, still night. If the temperature bo below freezing point 

 during the deposition of the dew, the particles of water freeze, 

 and hoar-frost is formed. 



THE PEROXIDE OP HYDROGEN (H a O a ). 

 It will be remembered that in procuring oxygen from the air, 

 one method was to pass a current of air over barytes, BaO, this 

 oxide, combined with another atom of oxygen, thus forming 

 BaO 2 . Upon raising the temperature, this peroxide of barium 

 parted with the second atom of oxygen, assuming its original 

 condition, BaO. But if we take this Ba0 2 , and dissolve it in 

 dilute hydrochloric acid, the following equation will indicate the 



BaO 6 + 2HC1 = BaCl a + H a O 2 ; 



that is, barium chloride and peroxide of hydrogen are formed 

 The barium chloride, BaCl 2 , is withdrawn from the solution, by 

 submitting it to the low temperature of a freezing mixture, at 

 which the BaCl 2 cannot remain in solution, and therefore it 

 " crystallises out," and is thus removed. More peroxide of 

 barium and hydrochloric acid are added to the solution, and the 

 process repeated until we get an appreciable quantity of the per- 

 oxide of hydrogen ; besides this, there will be a little barium 

 chloride in the solution ; by adding silver sulphate, a double 

 decomposition takes place, thus 



BaCl 3 + Ag a SO t = 2AgCl + BaSO, ; 



and seeing that both the products, silver chloride and barium 

 sulphate, are insoluble, they are precipitated. To separate tho 

 water from the peroxide of hydrogen, the mixture is placed 

 under the receiver of an air-pump the same arrangement as in 

 the freezing of water by its own evaporation, given in the last 

 lesson. The sulphuric acid absorbs the vapour formed by the 

 rapid evaporation, and the H 2 2 is left in the vessel. It is a 

 transparent syrupy liquid, possessing a peculiar odour, and 

 decomposes at even the low temperature of 20 Cent. ; at high 

 temperatures it decomposes with explosion. By its means fibrin 

 may be distinguished from albumen the two great components 

 of flesh ; the former decomposes it, the latter does not. It is 

 remarkable that finely-divided gold, platinum, or silver are 

 capable of decomposing this body into oxygen and water, with- 

 out undergoing any change themselves. Some protoxides, such 

 as that of lead, have the same effect, but in the process they 

 become higher oxides, whilst the oxides of the noble metals 

 not only are capable of causing the decomposition, but at the same 

 time lose their own oxygen, and are reduced to the metallic state. 

 This substance has a powerful bleaching action, and should a 

 process for producing it at a cheap rate be discovered, it would 

 prove a great boon, for it does not destroy the fabric it bleaches ; 

 and the result after the bleaching is accomplished being only 

 water, the necessity of great cleansing of the bleached material 

 is precluded. 



