This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
Thyme is the herbaceous part of Thymus vulgaris, a small under-shrub, growing wild in the South of Europe, and cultivated in our gardens. It has a characteristic, strong, agreeable odour, which it retains when dried, and a pungent, aromatic camphorous taste. These properties reside in a volatile oil (Oleum Thymi, U. S.), which is obtained, in the native districts of the plant, by distillation, and is said to be sent into commerce largely, under the name of oil of origan urn, by which it is often sold in the shops. Indeed, so commonly was it substituted for the oil of origanum, that the U.S. Pharmacopoeia, discarding entirely both the origanum and its oil, have adopted the oil of commerce with the name of oil of thyme, without, however, recognizing the plant itself in its officinal lists. It is lighter than water, and has at first a pale-yellow or greenish-yellow colour, which it gradually exchanges for a reddish-brown. As found in our shops, it is usually of a red colour, which is asserted to be that of the oil at the first distillation; the colourless oil being obtained by redistilling the red. I suspect, however, that, as just stated, the redness is the result of exposure, as in the case of so many other volatile oils. Thyme has the medicinal properties of the aromatics, but is much more used in cooking than in medicine.