SCIENCE IN SECONDARY SCHOOLS. 199 
studying and by the common acids. You find that the product from carbon 
has only a weak action but it seems to act in the same direction as the acids. 
Things which are similar may sometimes be substituted for one another, may 
they not? You know that limestone contains the gas which is derived from 
carbon and that the common acids in some way turn the gas out—will the 
acid product from phosphorus have a similar effect? Try! You thus discover 
that the two substances have similar properties, although not alike in strength 
—both are acidic substances. Are there any other non-metallic combustibles 
which you can study to ascertain if they yield acidic products? Although 
sulphur matches are not much used nowadays and almost the only occasion when 
sulphur is used in the house is when it is put into the dog’s water, you perhaps 
know the smell of burning sulphur. Burn some sulphur, pass the fumes into 
distilled water; taste the solution, test it with colours and add some chalk to 
it. You thus become acquainted with a third acidic product of combustion 
derived from a non-metal: the probability that non-metals form acid com- 
pounds and metals earths when associated with Fire air is therefore increased. 
Years ago, when it became desirable to give significant names to substances, 
the great French chemist Lavoisier introduced the name oxygen for the gas 
we have spoken of hitherto as Fire air; it retains this name to the present 
day, except among the Germans, who call it Sauerstoff, or sour-stuff—the stuff 
of which acids are made; but this is the meaning of the word oxygen, which 
is derived from two Greek words, oxus—acid and gennao—I produce. The 
compounds of oxygen are termed oxides and it may be mentioned here that the 
terminal ide is always restricted to substances which like those in question 
consist of only two others. 
Thus far you have been led to conclude that there are two kinds or classes 
of oxides—metallic and non-metallic: oxides of metals and oxides of non- 
metals. The latter it is found are acidic—they form acids when dissolved 
in water; except that the former are more or less earth-like in appearance, 
nothing has been observed which seems to be characteristic of these oxides as 
a class. Have you not noticed, however, that lime resembles the metallic 
oxides—is it perhaps a metallic oxide—what is characteristic of it: is it not 
its power of combining with carbonic gas and other acidic oxides—if then it be 
a metallic oxide, the metallic oxides generally may be expected to resemble it in 
combining with acidic oxides, may they not? You have found that not only is 
limestone acted upon by the common acids (muriatic acid, aquafortis and 
vitriolic acid) but lime also: in what way are they acted upon—comparing the 
effect of heat on limestone with that produced by acids, does it not seem that 
the lime in it is acted upon by the acid and the carbonic gas just let go? Does 
it not therefore seem desirable to study the action of the common acids on the 
metallic oxides generally in comparison with lime? 
But you will ask : what are these acids: how are they obtained? Surely, 
if we are to use them, we should know something about them. 
[Sketch history. of the discovery of oil of vitriol—pyrites used by palzolithic 
man—decay of and conversion into green vitriol and rust—distillation of green 
vitriol, production of oil of vitriol—strong sulphur smell, pyrites combustible, 
burning like sulphur but giving rust-like earth as well—preparation of vitriolic 
acid by burning sulphur, later with the aid of aquafortis.] 
Knowing what happens to sulphur when burnt, you will at once reason 
that vitriolic acid is in some way connected with the oxide you have prepared 
from sulphur—but you are told that it is formed from this oxide with the 
aid of air, water and aquafortis; or nowadays by passing the gas formed by 
burning sulphur together with air over heated finely divided platinum. Suppose 
you try this experiment. 
You will now realise that vitriolic acid consists of sulphur, oxygen and water, 
and that it is derived from an oxide which contains more oxygen than is con- 
tained in that formed on merely burning sulphur in air; this latter is a colourless 
gas, whilst the former is solid and forms a dense white smoke. To distinguish 
the two oxides, one is called sulphurous oxide, the other sulphuric oxide; 
whilst the acid formed from the one is called sulphurous acid and that formed 
from the other sulphuric acid, You know that von can associate sulphurous 
