GEOLOGY. 143 



almost always assailed by calumny, and who are 

 sometimes borne down by ingratitude, may in con- 

 sidering the benefits which they have rendered to 

 the human race, confidently appeal to heaven for 

 their reward, and to posterity (or their justifica- 

 tion. 



LETTER XXXIV. 



Western Region, Avgusi, 1820. 

 My Dear Sir, 



Although my luggage is small, yet I never 

 travel without a blow-pipe, and some tests and re- 

 agents. If in want cf any of the usual acids to 

 detect the presence of lime, I substitute strong 

 vinegar, which will generally produce an efferves- 

 cence, when poured on a calcareous substance. 

 If 1 am desirous of ascertaining the existence of 

 an alkali, or an acid, I stain paper with the petals 

 of a blue flower, and dip it in the water which is 

 to be tried, and if it turn green, it indicates an al- 

 kaline impregnation, and if red an acid one. If 

 .silver becomes black when thrown into water, it 

 denotes that sulphur is held in solution, and the 

 presence of iron is demonstrated, if the inner bark 

 ©f oak give water a dark appearance. With 



