EXPLANATIONS. 



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 Poggen- 

 dorfi's Annalen der Physik und Chemie, vol. 

 122 (1864) p. 626; or more completely in the 

 Bulletin de I'Acadeniie irtiperiale des Sciences 

 de St. Petersbourg, tome viii. (1869) p. 33. 



Measurements and W^eights.-The authors 

 regret to have been unable to adopt one standard 

 system of stating measurements. They have 

 mostly employed the English inch: the accom- 

 panying woodcut will facilitate its comparison 

 with the French decimal scale. The word milli- 

 metre is indicated in the text by the contraction 

 mm.; 7nicromillim,etre,sigmijuig the thousandth 

 part of a millimetre, and only used in reference 

 to the microscope, is abbreviated thus, mJcm. 



1 inch =: 



1 gallon = 



1 ounce (oz.) avdp. = 

 1 lb. avoirdupois = 

 1 cw^t. = 



1 ton 



1 kilo£:ramme 

 1 pecul 



25399 millimetres. 

 •io43 litres. 



2834 grammes. 

 = 453o9 „ 



= 112 lb. = 508 kilogrammes. 

 = 2240 „ = 1016 

 = 2 204 lb. avoirdupois. 

 = 133-33 lb. = 60-479 kilogrammes 



Thermometer. — The Centigrade Thermometer has been alone 

 adopted. The following table is given for compaHng the degrees of 

 the Centigrade or Celsius Thermometer with those of Fahrenheit's 

 Scale. 



