What you have sounds like the standard Epsom Salts . They are very commonly sold in "milk cartons" although the salts I buy at Walmart come in 4 pound reclosable bags. The main thing to look for when buying Epsom salt, at least in the U.S. are the words Magnesium Sulfate USP and/or Magnesium sulfate USP (heptahydrate) 100% this means you are getting a pure product without any additives and is safe to use as a laxative.