1911 Encyclopædia Britannica/Camphors

CAMPHORS, organic chemical compounds, the alcohols and ketones of the hydrocarbons known as terpenes, occurring associated with volatile oils in many plants. They are extracted together with volatile oils by distilling certain plants with steam, the volatile oils being subsequently separated by fractional distillation. The term “camphor” is generally applied to the solid products so obtained, and hence includes the “stearoptenes,” or solid portions of the volatile oils. They are mostly white crystalline solids, possessing a characteristic odour; they are sparingly soluble in water, but readily dissolve in alcohol and ether. Chemically, the camphors may be divided into two main groups, according to the nature of the corresponding hydrocarbon or terpene. In this article only the camphors of commercial importance will be treated; details as to the chemical structure, syntheses and relations will be found in the article Terpenes.

Menthol, mentha or peppermint camphor, C₁₀H₁₉OH, 5-methyl-2-isopropyl hexahydrophenol, an oxyhexahydrocymene, occurs in the volatile oils of Mentha piperita and M. arvensis (var. piperascens and glabrata), from which it is obtained by cooling and subsequently pressing the separated crystals; or by fractional distillation. It crystallizes in prisms, having the odour and taste of peppermint; it melts at 42° and boils at 212°. It is very slightly soluble in water, but readily dissolves in alcohol and ether. It is optically active, being laevo-rotatory. Menthol is used in medicine to relieve pain, as in rheumatism, neuralgia, throat affections and toothache. It acts also as a local anaesthetic, vascular stimulant and disinfectant.

Thymol, thyme camphor, C₁₀H₁₃OH, 3-methyl-6-isopropyl phenol, an oxycymene, occurs in the volatile oil of Ajowan, Carum ajowan, garden thyme, Thymus vulgaris, wild thyme, T. Serpyllum and horse mint, Monarda punctata. Thymol crystallizes in large colourless plates which melt at 44° and boil at 230°. It has the odour of thyme, is sparingly soluble in water, but very soluble in alcohol, ether and in alkaline solutions. In medicine it is used as an antiseptic, being more active than phenol. Iodine and potash convert it into di-iodthymol, which has been introduced in surgery under the names aristol and annidalin, as a substitute for iodoform.

Borneol, Borneo camphor or camphol, also known as Malayan, Barus or Dryobalanops camphor, C₁₀H₁₇OH, occurs in fissures in the wood of Dryobalanops aromatica, a majestic tree flourishing in the East Indies. This product is dextro-rotatory; the laevo and inactive modifications occur in the so-called baldrianic camphor. Borneol melts at 203° and boils at 212°. It is very similar to common or Japan camphor, but has a somewhat peppery odour. Sodium and alcohol reduce common camphor to a mixture of d- and l-borneol.

Common camphor, Japan or Laurel camphor, C₁₀H₁₆O, which constitutes the bulk of the camphor of commerce, is the product of the camphor laurel, Cinnamonum camphora, a tree flourishing in Japan, Formosa and central China. It also occurs in various volatile oils, e.g. lavender, rosemary, sage and spike. To extract the camphor, chips of the tree are steamed, and the mixed vapours of camphor, volatile oils and water are conducted to a condensing plant, where most of the camphor separates out. This is filtered, and the remainder, about 20% of the total, which is retained in solution, is extracted by fractional distillation and cooling the distillate. The crude camphor so obtained is exported from Japan in two grades—Samuel A and Samuel B. It is purified by mixing with a little charcoal, sand, iron filings or quicklime and subliming, by steam distillation or by crystallization. Common camphor forms a translucent mass of hexagonal prisms, melting at 175° and boiling at 204°. It sublimes very readily. In alcoholic solution it is dextro-rotatory; the laevo form, Matricaria camphor, occurs in the oil of Matricaria parthenium and closely resembles the d form. Camphor is chiefly used in the celluloid industry. The so-called “artificial camphor” is pinene hydrochloride (see Terpenes).

Externally applied it acts medicinally as a counter-irritant, and, in some degree, as a local anaesthetic, being also a definite antiseptic. It is, therefore, largely used in liniments for the relief of myalgia, sciatica, lumbago, etc. Combined with chloroform, thymol or carbolic acid, it is a valuable local application for neuralgia and for toothache due to dental caries. Taken internally, camphor is a nerve stimulant, a diaphoretic and a feeble antipyretic. It is excreted by the kidneys as various substances, including campho-glycuric acid (Schmiedeberg). In large doses it causes marked nervous symptoms, exhilaration being followed by abdominal pain, violent epileptiform convulsions, coma and death. Its internal uses are in hysteria, and in such conditions as diarrhoea, dysentery and cholera. It is a popular remedy for “cold in the head,” but it is not to be relied upon as a prophylactic against infection either by an ordinary cold or true influenza.