The acoustic properties of common salt render it an excellent medium for the transmission of sound, and as it possesses in a high degree the power of staying decomposition in dead organisms, it is, perhaps, the commonest of all preservatives. It is largely owing to its preservative property that common salt is an absolute necessity to the life of man and the higher animals, from a quarter to half an ounce a day being sufficient to prevent the putrefaction of food in the digestive tract in the case of an adult. In agriculture, salt is not only valuable as a destroyer of weeds and insect life, but used sparingly and with knowledge, it forms an excellent manure; while its more strictly chemical value in the manufacture of soda, chlorine, etc., causes it to play an important part in many branches of industry. Even at the highest temperatures, heat cannot effect the decomposition of common salt. At a red heat, pure sodic chloride melts and becomes liquid, and if cooled again, a solid crystalline mass is formed. Ordinary salt fuses at a lower temperature and volatilizes when heated in an open vessel. But even in a closed vessel the purest salt will volatilize at a white heat. When gases or fluids are present in the crystalline cavities, heat causes decrepitation. On the subject of the composition of brine, it is only necessary to add that it is so extremely variable that no two districts produce brine springs of the same strength and density, while the composition of ocean brine varies not only from ocean to ocean, but also for different parts and different depths in the same plane of water, and with the different distances from the mouths of large rivers. In the Cheshire district, the Brine test or Salinometer is graduated to show ounces in the gallon; but the gallon is the old Winchester Gallon of 231 cub. in. and not the Imperial Gallon of 277·274 cub. in. These are related to each other in the proportion of 10 to 12, therefore the Imperial Gallon will contain ⅕ more than the old gallon. Fully saturated brine by the Salinometer contains 42 oz. (2 lb. 10 oz.), therefore, in the Imperial Gallon 50·4 oz. As brines vary from 2 lb. 8 oz., or 40 oz. old measure, or 3 lb. or 48 oz. Imperial to 2 lb. 10 oz., or 3 lb. 2 oz. Imperial, so 1,000 gallons, which has been chosen as the measure for assessing brine-pumpers--under the Brine Pumping Compensation for Subsidence Act of 1891--will contain under the old measurement 2,625 lb. and under the Imperial 3,125 lb. of salt. CHAPTER II THE BEGINNINGS OF THE SALT INDUSTRY