ON SCIENCE IN SCHOOL CERTIFICATE EXAMINATIONS. 521 



certain districts where there is no rain to wash it away. Both kinds of salt- 

 petre are very valuable as manures. When vitriolic acid is added to saltpetre 

 and the mixture is gently warmed in a retort, a very volatile and acid liquid 

 distils over, the retort becoming full of brownish vapour. This liquid is very 

 corrosive, staining the skin a deep yellow. Of course, the alchemist* tried 

 the action of this acid on everything at hand, metals such as gold, silver, 

 copper, lead, tin, zinc and iron, finding that it dissolved all but gold : as it was much 

 stronger than the other acids they knew, they called it aquafortis. To the present 

 day, the jeweller uses aquafortis to distinguish spurious from real gc)ld. 



Aquafortis — or nitric acid as it is called on account of its formation from 

 nitre — you have learnt, is used in converting sulphurous into sulphuric acid ; 

 it must therefore be capable of giving off oxygen and must contain an oxide. 

 Nitre, or villainous saltpetre, as Hotspur calls it in Shakespeare's ' Henry IV.,' 

 has been used for centuries past in making gunpowder — a mixture of charcoal, 

 sulphur and nitre ; also in fireworks. The modern explosives — gun-cotton and 

 nitroglycerin — are also made with the aid of nitric acid. What happens when 

 gunpowder is fired — in what way do charcoal or sulphur and nitre interact? 

 Try to find out. 



Muriatic acid. — We get back to the kitchen and our own food once more 

 when we come to salt. Oil of vitriol acts upon it at once— fizzing takes place 

 and an acid fume escapes — spirit of salt, the old alchemists called it. They 

 were clever enough to find out that this fume is very soluble in water and 

 the solution is known to the present day by the oil-and-colour man, the plumber 

 and in kitchen regions, as spirit of salt. It is used in cleaning and removing 

 scale from baths, closet pans, etc. You will find that it is very acid and that 

 it stains the clothes but is not corrosive like oil of vitriol and aquafortis. The 

 plumber uses it in soldering, after ' killing it ' with zinc — everyone should 

 learn to solder and it may be worth your while to take the hmt given by the 

 plumber and see if you cannot follow up the clue. What is the action of the 

 oil of vitriol on the nitre and salt? You know that it displaces the carbonic 

 gas from limestone stuff — is its action on the salt and nitre a similar one — are 

 they comparable with limestone stuff ? 



The zinc, you find, is readily acted upon by the muriatic acid — examine the 

 product and compare it with similar substances which you have prepared pre- 

 viously ; it will be well to fit up apparatus which will enable you to prepare it 

 at will, at any desired rate. Contrasit it with coal gas and determine very 

 carefully what is formed from it v^hen it is burnt. 



When this inquiry is complete, you should recognise that you have made 

 a discovery of the greatest importance with reference to your previous work 

 and to the nature of foodstuffs such as starch. Again, you have an illustration 

 of the fact that information is to be gained from the most unexpected quarters 

 — who would suppose that the plumber could help you to determine the com- 

 position of starch? 



NATuaE OF Water. 



You believe that you have obtained this clue to the composition of water — 

 that it consists of the gas which is called water-stuff or hydrogen (because it affords 

 water when burnt) and oxygen : as you know that all other things which you have 

 burnt combined with the oxygen. Still nothing must be taken for granted in our 

 vvork : it is possible that the oxygen in air is not alone concerned ; cannot 5'ou devise 

 some method of using oxygen in a form in which there can be no doubt that if water 

 be obtained it is formed from oxygen and hydrogen alone ? How did you burn 

 carbon with oxygen alone ? 



You are now satisfied that you have established the fact that water con- 

 sistB of hydrogen and oxygen. Is it not worth while to submit the oxides 

 generally to the action of hydrogen? Will you not be able to test lime if 

 you find that they all give up their oxygen to hydrogen? The results enable 

 you to classify the metallic oxides in two groups; although you have not yet 

 solved the problem regarding lime, have you not narrowed it— is it not clear 

 that if it be a metallic oxide it is the oxide of a metal of a particular kind? 



Perhaps by studying the action of spirit of salt, which dissolves oxides, it 



