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mass will be so much impregnated with nitre, that one pound of it will yield two ounces of the salt. The King of Prussia was early sensible of the importance of a Salt-petre manufactory in his do- minions, and has therefore for this purpose always obliged his farmers to build their fences of com- mon earth, mixed with a quantity of straw and dung. These substances corrupt in the course of a few ears. The fences are then shaved, or wholly taken down, and afford a large quantity of Salt- petre. The sweeping of the streets of a single village in Hanover, afford all the Salt-petre that is used in the whole of that Electorate. Dr. Franklin, from whom I received this piece of of information, assured me that the manufactory of salt from the above materials was so simple that it was carried on entirely by an illiterate old man and his wife. The greatest part of the Salt-petre that comes from India is obtained nearly in the same manner. It is prepared entirely from the offals of the city of Patna. Mr. La Roux, of the academy of Rouen, informed me, that the following receipt had been found to answer very well in many parts of France for the manufactory of this salt : it is taken from Glauber, a German Chemist. I shall deliver it in his own words: "I will shew a way to such as have no inhe-
ritance left them from their parents, nor have any
thing
accompanied with moisture, which render them improper for the formation of Salt-petre. The house which contains our Salt-petre materials, should therefore, in this country, be exposed to the north-west winds.
