Once more, there is a compound of nitrogen and oxygen which presents the appearance of a deep-red, almost black gas; there is also a compound of nitrogen and oxygen which is a clear, colourless gas; yet both contain the same elements united in the same proportions. But a detailed consideration of _isomerism_, _i.e._ the existence of more than one compound built up of the same amounts of the same elements yet possessing different properties, would lead us too far from the main path of chemical advance which we wish to trace. The chemist is to-day continually seeking to connect the properties of the bodies he studies with the molecular structures of these bodies; the former he can observe, a knowledge of the latter he must gain by reasoning on the results of operations and experiments. His guide--the guide of Lavoisier and his successors--is this: "Similarity of properties is associated with similarity of composition"--by "composition" he generally means molecular composition. Many facts have been amassed of late years which illustrate the general statement that the properties of bodies are connected with the composition of those bodies. Thus a distinct connection has been traced between the tinctorial power and the molecular composition of certain dye-stuffs; in some cases it has even become possible to predict how a good dye-stuff may be made--to say that, inasmuch as this or that chemical reaction will probably give rise to the production of this or that compound, the atoms in the molecule of which we believe to have a certain arrangement relatively to one another, so this reaction or that will probably produce a dye possessed of strong tinctorial powers. The compound to the presence of which madder chiefly owes its dyeing powers is called _alizarine_; to determine the nature of the molecular structure of this compound was, for many years, the object of the researches of chemists; at last, thanks especially to the painstaking zeal of two German chemists, it became fairly clear that alizarine and a compound of carbon and hydrogen, called _anthracene_, were closely related in structure. Anthracene was obtained from alizarine, and, after much labour, alizarine was prepared from anthracene. Anthracene is contained in large quantities in the thick pitch which remains when coal-tar is distilled; this pitch was formerly of little or no value, but as soon as the chemical manufacturer found that in this black objectionable mass there lay hidden enormous stores of alizarine, he no longer threw away his coal-tar pitch, but sold it to the alizarine manufacturer for a large sum. Thus it has come to pass that little or no madder is now cultivated; madder-dyeing is now done by means of alizarine made from coal-tar: large tracts of ground, formerly used for growing the madder plant, are thus set free for the growth of wheat and other cereals.