and with only three-fourths of the pressure of air upon it that we have at the sea-level. The lake-water thus concentrated is run into smaller tanks. It contains carbonate and sesquicarbonate of soda, and common salt. The addition of lime converts the sesquicarbonate of soda into simple carbonate, and this is separated from the salt by taking advantage of their different points of crystallization. The salt is partly consumed, and partly used in the extraction of silver from the ore, and the soda is bought by the soap-makers.
Humboldt’s remarks on the small consumption of salt in Mexico are curious. The average amount used with food is only a small fraction of the European average. While the Tlascalans were at war with the Aztecs, they had to do without salt for many years, as it was not produced in their district. Humboldt thinks that the chile which the Indians consume in such quantities acts as a substitute. It is to be remembered that the soil is impregnated with both salt and natron in many of these upland districts, and the inhabitants may have eaten earth containing these ingredients, as they do for the same purpose in several places in the Old World.