The fixity of composition of chemical compounds has now been established; a definite meaning has been given to the term "element;" the conception of "atom" has been gained, but much remains to be done in the way of rendering this conception precise; and fairly good, but not altogether satisfactory methods have been introduced by which the relative weights of the atoms of elements and compounds may be determined. At this time chemists are busy preparing and describing new compounds, and many new elements are also being discovered; the need of classification begins to be felt more and more. In the days of Berzelius and Davy strenuous efforts were made to obtain some generalizations by the application of which the many known elements and compounds might be divided into groups. It was felt that a classification might be founded on the composition of compounds, or perhaps on the properties of the same compounds. These two general principles served as guides in most of the researches then instituted; answers were sought to these two questions: Of what elements is this compound composed? and, What can this compound do; how does it react towards other bodies? Lavoisier, as we know, regarded oxygen as the characteristic element of all _acids_. This term _acid_ implies the possession, by all the substances denoted by it, of some common property; let us shortly trace the history of this word in chemistry. Vinegar was known to the Greeks and Romans, and the names which they gave this substance tell us that sourness was to them its characteristic property. They knew that vinegar effervesced when brought into contact with chalky earths, and that it was able to dissolve many substances--witness the story of Cleopatra's draught of the pearl dissolved in vinegar. Other substances possessed of these properties--for instance oil of vitriol and spirits of salt--as they became known, were classed along with vinegar; but no attempts were made to clearly define the properties of these bodies till comparatively recent times. The characteristics of an acid substance enumerated by Boyle are--solvent power, which is exerted unequally on different bodies; power of turning many vegetable blues to red, and of restoring many vegetable colours which had been destroyed by alkalis; power of precipitating solid sulphur from solutions of this substance in alkalis, and the power of acting on alkalis to produce substances without the properties of either acid or alkali. But what, one may ask, is an alkali, of which mention is so often made by Boyle?