Pharmacographia / A history of the principal drugs of vegetable origin, met with in Great Britain and British India

Polarization.—Most essential oils, and the solutions of several substances described in this book are capable of effecting the deviation of a ray of polarized light. The amount of this rotatory power cannot be regarded as constant in essential oils, and is greatly influenced by various causes. As to alkaloids and other organic compounds, the deviation frequently depends upon the nature and quantity of the solvent. The authors have thought it needful to record in numerous cases the results of such optical investigations, as determined by means of the Polaristrobometer invented by Wild, and described in Poggendorff’s Annalen der Physik und Chemie, vol. 122 (1864) p. 626; or more completely in the Bulletin de l’Académie impériale des Sciences de St. Pétersbourg, tome viii. (1869) p. 33.

Measurements and Weights.—The authors regret to have been unable to adopt one standard system of stating measurements. They have mostly employed the English inch: the accompanying woodcut will facilitate its comparison with the French decimal scale. The word millimetre is indicated in the text by the contraction mm.; micromillimetre, signifying the thousandth part of a millimetre, and only used in reference to the microscope, is abbreviated thus, mkm.

Thermometer.—The Centigrade Thermometer has been alone adopted. The following table is given for comparing the degrees of the Centigrade or Celsius Thermometer with those of Fahrenheit’s Scale.

THERMOMETRIC TABLE.