Leaving, however, these questions of origin and derivation, let us come to that of the chemical doctrine taught in the book which Michael Scot compiled, or at least translated. The title of the _Liber Luminis Luminum_ is a significant one, and has a real relation to the contents of the work itself.[135] To discover the sense which it must be held to bear we have only to turn to the passage in which, speaking of alum, the author says: ‘sicut illuminat pannos, ita illuminat martem ut recipiat formam lunae. Ut enim lana illuminatur ita et metalla illuminantur.’[136] A distinction is clearly present in the writer’s mind between the substance and the form of the metals. He probably held that there existed but one common metallic substance, which assumed the appearance of iron, gold, or silver, according to the form which it had received. His employment of the title _Liber Luminis Luminum_ was meant to indicate that the purpose of his book was that of teaching the student how metals might best be purified and improved. Their inferiority, when of the baser kind, he conceived as an impurity, manifesting itself in the imperfect forms of lead, iron, tin, and copper. He believed that this being removed or changed by art, they might be made to shine with the lustre and indeed possess the only distinctive quality of gold and silver. That we have rightly read the meaning of this title seems plain from a curious spelling which may be noticed in the _Liber Dedali_. ‘Illuminantur’ there appears as ‘aluminantur.’ The chemistry taught in these books did in fact prescribe the use of alum as a great means of purifying and refining the metals. The preface of the _Liber Luminis_ closes with a brief summary of the chapters which compose the work itself. The first of these deals with the different salts used in this chemistry: common salt; rock salt; alkali; sal ammoniac; nitre and others. The second treats in like manner of the various kinds of alum, the third describes the vitriols, and the fourth the powders or spirits, by which we are to understand those minerals which are capable of being sublimed or made volatile, such as sulphur, arsenic, and mercury. Two supplementary chapters, the one on the preparation of the salts, alums, and vitriols, and the other on that of the remaining class of chemicals, complete the whole book. This supplement seems genuinely such, as it is not mentioned in the general contents, as these appear in the preface. Perhaps we do not err if we suppose it to have embodied the result of Scot’s own experiments in alchemy.