DESCRIPTION OF A NITRE CAVE. 243 



nitre daily, but forty pounds may be considered as the average 

 product of the labour of three men at those works which I had 

 an opportunity of visiting. 



The workmen being badly provided with tools and appara- 

 tus, desert a rock whenever its size or hardness renders it diffi- 

 cult for them to manage it, and go in quest of a new establish- 

 ment. Several caves and rocks which these strolling chemists 

 have deserted, still contain many thousand pounds of nitre. 

 These men are continually searching for masses of pure nitre, 

 or rich veins of ore, by which much of their time is unprofita- 

 bly dissipated. Still however-most of our saltpetre-makers find 

 it their interest to work the sand rock rather than the calcareous 

 caverns, which last yield a mixture of nitrate of pot-ash and 

 nitrate of lime. The rock saltpetre is greatly preferred by our 

 merchants and powder-makers, and commands a higher price. 



Mr. Barrow, in his travels through the southern parts of the 

 continent of Africa, discovered native nitre, which is probably 

 similar to the rock saltpetre of Kentucky. But Bowles, Dillon 

 and Townshend assure us that those districts in Spain, which 

 afford nitre most abundantly, contain neither chalk, limestone, 

 gypsum, nor any other calcareous substance. The nitrate of 

 pot-ash is obtained there by filtrating a certain kind of black 

 mould which will continue for ages to yield annual supplies 

 of it, together with muriate of soda, sulphate of magnesia, 

 nitrate and sulphate of lime. Here then appears to be such a 

 relation existing between the different saline substances, both 

 acids and alkalies, that the causes which produce one of them, 

 owing to some yet undiscovered circumstance, regularly pro- 

 duce all the rest. According to these authors the same mould 

 will continue forever to yield these salts annually. This obser- 

 vation if correct, would induce us to believe, that both acids 

 and alkalies are wholly formed from atmospheric air and not 

 from the soil; as the soil would certainly be exhausted if any 

 considerable portion of it entered into the composition of either 

 the acids or alkalies, and would soon lose its power of attract- 

 ing from the air the other constituent principles of the salts. 

 Both in Spain and India, we are informed, that the mould 

 which for iit'ty years in succession has yielded nitre, still con- 



