254 DR. CHARLES A. BURGHARDT ON THE 



radiating foliating masses, or in bundles of acicular crystals 

 which are characterized by a fine vertical striation upon 

 their vertical faces. Globular, reniform, and stalactitic 

 atacamite is also known. Thus it is safe to infer that in 

 a great majority of cases the mineral has been formed in 

 some way from a solution of copper-salt, and not by 

 igneous action. It is also a well-known fact that ataca- 

 mite crusts are occasionally found resting upon clefts in 

 the lava of some volcanoes, being formed by the action of 

 hydrochloric acid gas evolved by the volcano upon copper 

 or a compound of that metal ; but these crusts are insig- 

 nificant in quantity and comparatively scarce. In Chili, 

 Bolivia, and the west coast of Africa very large quantities 

 of atacamite are obtained as a commercial product ; it 

 therefore becomes interesting to the mineralogist to as- 

 certain under what conditions these formations arose ; and 

 in order to do this, a thorough chemical examination of 

 the mineral itself is necessary. Atacamite is known to 

 occur in nature in three states of hydration ; and as an 

 example of the first I may mention a specimen from Algo- 

 don Bay, Bolivia, analyzed by Bibra (Jahresbericht, 1858, 

 p. 740), which had the following composition, viz. : — 



per cent. 



Cu 59-25 



CI 1611 



O 12-51 



HjO 1213 



lOO'OO 



the formula corresponding with the above analysis being 

 Cu^OjCU + sH.O. 



The atacamite of Copiapo, Chili, as analyzed by Field 

 (Journ. Chem. Soc. [7] iii. 193) has the following compo- 

 sition, viz. : — 



