BOOK XII. 



561 



A — Vat in which the soda is mixed. B — Caldron. C— Tub in which chrysocolla is 



CONDENSED. D — COPPER WIRES. E — MoRTAR. 



Saltpetre* is made from a dry, slightly fatty earth, which, if it be re- 

 tained for a while in the mouth, has an acrid and salty taste. This earth, 

 together with a powder, are alternately put into a vat in layers a palm deep. 

 The powder consists of two parts of unslaked lime and three parts of ashes of 

 oak, or holmoak, or Itahan oak, or Turkey oak, or of some similar kind. Each 

 vat is filled with alternate layers of these to within three-quarters of a foot 

 of the top, and then water is poured in until it is full. As the water percolates 

 through the material it dissolves the saltpetre ; then, the plug being pulled 

 out from the vat, the solution is drained into a tub and ladled out into small 



•Saltpetre was secured in the Middle Ages in two ways, but mostly from the treatment 

 of calcium nitrate efflorescence on cellar and similar walls, and from so-called Scdtpetre 

 plantations. In this description of the latter, one of the most essential factors is omitted 

 until the last sentence, i.e., that the nitrous earth was the result of the decay of organic or 

 animal matter over a long period. Such decomposition, in the presence of potassium and 

 calcium carbonates — the lye and lime — form potassium and calcium nitrates, together with 

 some magnesium and sodium nitrates. After lixiviation, the addition of lye converts the 

 calcium and magnesium nitrates into saltpetre, i.e., Ca (N0»)« + K^CO' = Ca C0» + 2KN0». 

 The carbonates precipitate out, leaving the saltpetre in solution, from which it was evaporated 

 and crystallised out. The addition of alum as mentioned would scarcely improve the 

 situation. 



The purification by repeated re-solution and addition of lye, and filtration, would 

 ehminate the remaining other salts. The purification with sulphur, however, is more diificult 



