alkali vs salt water

alkali

noun
  • Soluble mineral matter, other than common salt, contained in soils of natural waters. 

  • One of a class of caustic bases, such as soda, soda ash, caustic soda, potash, ammonia, and lithia, whose distinguishing characteristics are dissolving in alcohol and water, uniting with oils and fats to form soap, neutralizing and forming salts with acids, turning to brown several vegetable yellows, and changing reddened litmus to blue. 

salt water

noun
  • any water containing dissolved salt; brine 

How often have the words alkali and salt water occurred in a corpus of books? (source: Google Ngram Viewer )