X, A, 6 Cox and Dar Juan: Salt Industry and Resources 385 



water and precipitating the silica, oxide of iron, calcium carbo- 

 nate, and calcium sulphate. 



The reservoir in which the first evaporation takes place is 

 usually a fish pond; in addition to this, there are in most salt 

 plants three rows of shallow concentration reservoirs, seldom 

 less than four in a series, and often six or seven. The brine is 

 drawn from one reservoir to another as it strengthens and de- 

 creases in volume by evaporation, and new water is in turn 

 admitted from the bay. Beyond the reservoirs crystallizing 

 ponds are constructed in the manner already described. See- 

 page from the crystallizing ponds is collected in ditches, which 

 carry it to a well, from which it is baled out with a bamboo 

 sweep into another ditch, which returns it to the evaporation 

 reservoirs containing the strongest brine. When the crystalliz- 

 ing ponds are higher than the evaporation reservoirs the brine 

 is dipped up by hand ; sometimes it is poured into an apparatus 

 similar to that used in filling the leaching vats, allowed to run 

 through a straw filter, and thus transferred to the crystallizing 

 ponds. When land above tide level is employed for the greater 

 part of the manufacturing plant, the water is elevated with a 

 bamboo sweep. In the ideal plant the whole process is by 

 gravity. 



Water transportation connects most or all of the salt works 

 with deep water, from where connection can be made via navi- 

 gable streams with many of the inlands provinces. 



The working season for the plants along Manila Bay varies 

 somewhat from year to year, but usually begins in December 

 and continues until about May — a period of approximately one 

 hundred fifty days. 



The product obtained by the process above described is coarse 

 and not usually of the best quality, as it contains magnesium 

 salts and other impurities. The brine thus treated will not give 

 a product containing much over 93 to 94 per cent sodium chloride. 

 If, however, the mother liquor containing the bulk of impu- 

 rities — that is, most of the magnesium and sodium sulphates and 

 practically all the magnesium and calcium chlorides — is removed 

 from time to time, a much higher grade of salt may be produced. 



The magnesia and lime content of Philippine salt are shown 

 in Table II. 



The first concentration or evaporation reservoirs are preceded 

 by a fish pond, not shown in figure 5, specific gravity 1.025, 

 which is supplied at its intake with water having a specific 

 gravity of 1.024. In all of the Philippine plants the brine was 

 transferred from the evaporation reservoirs to the crystallizing 



