ethylene—which, is the chief light-giving constituent of illuminating gas. Ethylene, when brought into contact with sulphuric acid, forms a liquid combination, and this when treated with potassium hydrate is converted into alcohol. Having thus built up from its elements a substance formerly known only as a product of fermentation, we may proceed at once to decompose it again into its elements. We can easily regain the carbon which it contains, by heating alcohol with sulphuric acid, which again converts it into ethylene; and this gas, when mixed with chlorine gas and lighted, burns away, leaving carbon, which as a dense black smoke fills the vessel.
An event very encouraging and helpful to synthetical investigations was the artificial preparation of urea, a product of secretion in animal bodies, resulting from the decay of muscle, and one of the most important substances in animal exchange of matter. When Woehler, in 1828, found out that, by a chemical process, it can be composed with all its physical and chemical properties, this event gave a tremendous shock to the foundations of the doctrine formerly believed, that a "vital power" governed the functions of the organs of living animals, independently of physical as well as of chemical forces. The discovery of artificial urea was followed by others in an uninterrupted series, which, besides the practical interest they were entitled to claim, threw a new and clear light upon many processes in organic life. In glancing at some of them, we confine ourselves to cases of more general interest.
A conspicuous instance of the degree to which synthetical chemistry has enabled us to imitate nature in some of the processes going on in the bodies of plants and animals is represented by the changes which salicin undergoes. It is to this white and crystalline compound—belonging to the chemical group of glucosides—that the leaves of willow and poplar trees owe their bitter taste. Several species of Spirœa, while young, also contain salicin, which, during growth, is converted into a volatile oil of reddish color—salicylic aldehyde—an oil which, remarkably enough, is also produced from salicin in the body of the larvæ of Chrysomela populi, a beetle feeding on the leaves of poplar-trees. In Spirœa, as well as in other plants containing this oil, it is partly transformed into salicylic acid, which in its turn in Gaultheria procumbens and Betula lenta combines with methyl to form a product known as "wintergreen-oil." Now by synthesis we can artificially reproduce all these changes, though pursuing quite a different way from that which Nature follows. We can convert salicin into salicylic aldehyde; we can transform this into salicylic acid, and we can produce wintergreen-oil by combining this acid with methyl. We can even manage to prepare salicylic acid and wintergreen-oil from coal-tar, a substance which, as everybody may
