=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.
[Illustration]
=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._
1 inch = 25·399 millimetres.
1 gallon = 4·543 litres.
1 ounce (oz.) avdp. = 28·34 grammes.
1 lb. avoirdupois = 453·59 ”
1 cwt. = 112 lb. = 50·8 kilogrammes.
1 ton = 2240 ” = 1016 ”
1 kilogramme = 2·204 lb. avoirdupois.
1 pecul = 133·33 lb. = 60·479 kilogrammes.
=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_.