How to make a Thorn- house 441 



nowise impoverished as regards its desired ingredient. The 

 twigs are encrusted only, not really fosillised, the difference 

 being that the lime is not taken into the substance of the 

 wood. It is the same at Knaresbrough. 



The above description of a simple but ingenious expedient 

 for obtaining salt cheaply may serve to impress important 

 facts in natural science. The chief of these is that salt water 

 when evaporated leaves behind its salt. This is the clue to 

 formation of salt marshes, salt inland seas, and if we go back 

 to geological times, to the deposit of beds of rock salt. The 

 great American salt lakes have been formed in this way, and 

 the Dead Sea, whilst it is probably supplied chiefly by the 

 solution of adjoining rocks, also owes much to evaporation of 

 the waters brought down by the Jordan. The water resulting 

 from such evaporation is pure if the process occur in still air, 

 but may easily contain salt if there is wind. To the owners 

 of a thorn-house a current of air is essential, but a high wind 

 robs them of their .salt. Many other ingenious (more or less) 

 methods are in use for obtaining from sea water and other 

 sources this great necessary of life. The principle in all is 

 the encouragement of evaporation. In some instances the 

 object is to change salt or brackish water into fresh. It is 

 scarcely needful to remind even our youngest readers of the 

 facts that the salt in water is in solution only, not chemical 

 combination, and that all surface waters, even rain water 

 itself, contains minute quantities of salt. It is from this 

 source that many inland salt lakes obtain their supply. 



An account of the various methods by which in the past 

 salt has been obtained for domestic use, might constitute a 

 very important chapter in the history of social progress. 

 Although, in minute quantities, present almost everywhere, 

 the problem of how to procure it has taxed the ingenuity of 

 many a primitive race. It was one of the chief articles of 

 early international traffic, and some of the first trade routes 

 of which we have any knowledge were established for the 

 conveyance of salt. 



