520 REPORTS ON THE STATE OF SCIENCE, ETC. 



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 ; this' too 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 palaeolithic 

 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. 1 



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 ; 

 whibt the acid formed from the one is called sulphurous acid and that formed 

 from the other sulphuric acid. You know that you can associate sulphurous 

 oxide with lime and that you can displace carbonic gas from limestone by 

 sulphurous oxide and also by phosphoric oxide ; as you also know that sulphuric 

 acid acts on limestone, you will be prepared to argue that sulphuric oxide can 

 also combine with lime. Phosphoric oxide has proved to be stronger than 

 sulphurous oxide — try whether sulphuric or sulphurous oxide be the stronger, 

 in a similar sense. 



Contrast sulphurous with sulphuric acids. The fact that sulphuric oxide 

 proves to be the stronger is clearly of interest in justification of the name 

 sour-stuff or oxygen : the stronger and more pronounced acid being that which 

 contains the major proportion of oxygen. 



Aquafortis, — There is no doubt that, in early times, as soon as the alchemists 

 found a new substance, they tried its effect on all the substances with which 

 they were acquainted. In this way, when they discovered oil of vitriol, besides 

 finding out more or less by accident if not by carelessness that it was very 

 corrosive and destructive of their skin and clothes, they probably very soon 

 tried what action it would have on sub.^tances such as nitre or saltpetre and 

 sea salt. The former often appears in the form of crystals on the soil in the 

 neighbourhood of manure heaps ; saltpetre occurs in large quantities in' Chili in 



