On Darwinite, a new Mineral Species from Chile. 423 



by the annexed figure. 

 This is, in fact, the 

 form of the curve in 

 the case considered by 

 Plucker, where the line 

 Iisat infinity, thepoints 

 of contact being the 

 middle points of the 

 sides. And his five 

 groups of curves, a, (3, 

 7, B, e, and two subdi- 

 visions of the group /3 

 (see pp. 221-224), cor- 

 respond to the following positions of the line in regard to the 

 triangle and curve, viz. — 



a. The line cuts the three sides produced. 



/3. It passes through an angle, {a) cutting, or (b) not cutting 

 the curve. 



7. It cuts two sides and a side produced, but does not cut or 

 touch the curve. 



6\ It cuts two sides and a side produced, and touches the 

 curve. 



€. It cuts two sides and a side produced, and cuts the curve. 



It is hardly necessary to remark that, in the general case, the 

 tangential equation of the curve is 



or what is the same thing, 



and that the curve is therefore of the sixth class. 



2 Stone Buildings, W.C., 

 October 16, 1860. 



LVII. On Darwinite, a new Mineral Species from Chile, 

 By David Forbes, F.R.S. $c* 



1THIS mineral, a specimen of which was given me as being 

 native arsenic, is stated to occur near Potrero Grande, not 

 many miles to the south-east of the town of Copiapo in North- 

 ern Chile, where it is said to present itself as small veins or 

 strings cutting through the porphyritic claystones which form 

 the mountain range at that place, and which represent the upper 

 oolitic formation in geological age. 



* Communicated by the Author. 



