204 



THE POPULAR EDUCATOR. 



get zinc perfectly pure ; 



volume of the oxygen. The reason why there is a little too 

 much hydrogen is that oxygen is more soluble in water than 

 hydrogen, and as the gases rise through the water more oxygen 

 is retained than hydrogen. 



It was said in the last lesson that the escape of hydrogen 

 from the surface of zinc when in acidulated water was really 

 due to galvanic action. It is a most difficult thing to 

 the ordinary metal, the sur- 

 face is com- 

 posed of par- 

 ticles of pure 

 and of impure 

 zinc. If now 

 any two differ- 

 ent metals be 

 acted upon by 

 dilute acid, 

 current of elec- 

 tricity will be developed, so that 

 over the zinc surface there are 

 thousands of " galvanic couples." 

 A particle of pure metal and its 

 neighbour of impure metal form- 

 ing a " couple," between the two 

 a current begins to circulate, and 

 as it passes the water is decom- 

 posed the hydrogen escapes from 

 one particle and the oxygen at- 

 ^= tacks the other. 



If zinc be amalgamated that 

 H is, if the surface be rubbed with 

 mercury these particles are 

 _ formed into a paste, and inti- 

 | mately mixed, so that the whole 

 surface becomes impure, and 

 therefore amalgamated zinc will 



Fig. 27. 



not liberate hydrogen. For this reason the zinc plates in gal- 

 vanic batteries are always amalgamated. 



The other method of getting hydrogen in large quantities 

 will suggest at once another means of analysing water. If the 

 porcelain tube and its contents, the iron turnings, be carefully 

 weighed, and also the water in the flask, and then the tube, 

 after the whole of the water has been sent' over the red-hot 

 turnings, be also weighed, the difference will evidently be the 

 weight of the oxygen, which has combined with the iron, or the 

 weight of the oxygen which was in the water ; the difference 

 between this and the whole weight of the water will give the 

 hydrogen. 



It will at once occur to the student that the " reduction " of 

 black oxide of copper by hydrogen offers a fourth means of 

 determining the constitution of water. By weighing the 

 "reduction tube" and its contents before and after the ex- 

 periment, the weight of oxygen given off by the cupric oxide 

 will be ascertained, and also the weight of the water formed 

 be known. It will also be found that 88'89 parts of oxygen 



by weight unite 

 with 11-11 parts 

 of hydrogen, to 

 form 100 of water. 

 The oxygen and hy- 

 drogen, in entering 

 into combination to 

 form water, pro- 

 duce a very high de- 

 gree of heat. This 

 has been taken 

 advantage of in 

 the " oxy-hydrogen 

 blow-pipe." Two 

 strong india-rubber 

 bags are filled with 

 the gases, which are 

 carried by separate tubes to the burner. The mixed gases issue 

 through a fine nozzle, and when lit they burn with a pale non- 

 luminous Same, which has a very high temperature : wire, 

 watch-springs, etc., are melted immediately, and in many 

 instances the metal scintillates beautifully. 



When the jet is directed on a piece of lime, the lime is raised 



to such a white heat as to produce one of the most brilliant of 

 lights the oxy-hydrogen light. The best arrangement is the 

 one sketched in Fig. 29. In this manner it is used for magic- 

 lanterns ; the stand sliding into the bottom of the lantern, the 

 handles, D and c, remaining outside. Instead of hydrogen, ordi- 

 nary coal-gas is generally used. The greatest care must be 

 taken after each exhibition to empty the bags, H, o, lest, when 

 they are re-filled, the oxygen should by any chance be put into 

 the hydrogen-bag, when a very violent explosion would be the 

 consequence. 



The gases mix in B, a brass tube, which is packed with fine 

 wire gauze. The cylinder of lime, L, is supported on a wire 

 which is threaded with a screw which passes through a collar 

 soldered between the two tubes. By this wire the lime can be 

 turned or raised, for after a time the jet burns a hole in the 

 lime. 



For a further precaution it is well to have a differently shaped 

 handle, c, for the oxygen. It is possible to burn the gases mixed 

 from one bag, through Hemming's Safety Jet, which is simply a 

 brass tube packed with discs of fine wire gauze ; but the flame 

 has been known to pass down through forty of these discs before 

 it was extinguished by their cooling action. It is therefore 

 more safe to burn the gases from separate bags. 



If we follow water in passing through its three states, we 

 shall discover many strange properties. If a piece of ice be 

 placed in a vessel over a fire, with a thermometer, the thermo- 

 meter will rise or fall until it reaches Cent., when the ice will 

 begin to melt. Although heat is continually passing from the fire 

 into the vessel, yet the thermometer will remain at until the 

 last particle of ice has disappeared. Where, then, has all this 

 heat gone ? It has gone to do a certain work, to turn the solid 

 into a liquid, and because this heat does not affect the thermo- 



meter it is called " latent heat." To measure this " latent 

 heat" we must have some standard. We cannot speak of a 

 metre or a litre of " caloric ; " therefore the unit of heat, or the 

 measure of heat, is that amount of heat which is required to raise 

 a unit of water 1 Cent. The latent heat of water is 79 of these 

 thermal units ; that is to say, to melt a kilogramme of ice, 

 which is at Cent., requires as much heat as it does to raise 

 79 kilogrammes of water 1. 



Every solid upon assuming the liquid condition renders latent 

 a certain quantity of heat ; and every liquid becoming a solid 

 gives out its latent heat. 



Let sodium sulphate be added to boiling water until the water 

 will dissolve no more, and then allow the water to cool ; if un- 

 disturbed the salt will not crystallise, but upon stirring it with 

 a thermometer, a large quantity of the Glauber salt will assume 

 the solid state, and a rise in the temperature will be indicated 

 by the thermometer. 



The fact that a solid cannot pass into a liquid without ren- 

 dering " latent ' ' a certain quantity of heat, has been taken advan- 

 tage of to form freezing mixtures. Whenever a solid is so acted 

 upon by another solid or a liquid as to be compelled to assume a 

 liquid condition, heat must be taken from surrounding bodies. 

 Snow, or powdered ice, and salt liquefy each other, and produce 

 the temperature of Fahrenheit's zero. Ammonium nitrate and an 

 equal weight of water will offer an example of a freezing mixture 

 in which one of the bodies is already in a liquid condition. If 

 ither of these mixtures be made in a test tube, moisture will 

 first condense on the outside and then freeze. If ice be required 

 in some quantity, a thin dish containing water is placed in the 

 mixture. 



In the raising of the temperature of water from 0, a very 

 remarkable exception to an otherwise universal law is observed. . 

 All bodies expand when heated; but upon heating water at 

 Cent, it contracts until it reaches 4 Cent., and from this point 



