366 PHILOSOPHICAL TRANSACTIONS. [aNNO 1753. 



centres c and d are drawn off to the distance c d, and the points a and b are two 

 objects, or different parts of the same object ; therefore the lines a c g and b d g 

 represent two rays that pass through the centres or poles of the segments, and 

 are therefore not at all refracted, but go straight through to g, where they in- 

 tersect ; and g being the respective focus to the distance of the objects from the 

 glass, the two images will coincide at that point. It appears from the figure, 

 that ab: cd:: gh: ge; and from a common proportion in optics, g h : g e : : 

 he: e f. Therefore ab: Cd:: he: ef; f being the focus of parallel rays ; 

 and consequently the angles a e b and c f d are equal. That is, the angle sub- 

 tended by the distance of the centres of the segments from the distance of the 

 focus of parallel rays, is equal to the angle subtended by the distance between 

 the objects a and b from the end of the telescope. 



XXVIIl. On the Copper Springs in IVichlow in Ireland. By John Bond, 



M. Z). p. 181. 



A spring of water flows from a rich copper mine, and is of a sharp acid taste, 

 and light-blue colour. It is received and collected in pits, where iron bars are 

 placed, which, after lying in the water about 3 months, are entirely consumed, 

 and at the bottom of the pits, a quantity of copper, greater than that of the iron 

 is found, in the form of coarse sand. This fact is confirmed by profitable expe- 

 riments, often repeated since the discovery, the honour of which is due to Mr. 

 Matthew Johnston, a worthy old gentleman, and one of the proprietors of the 

 mine, who first proposed this method of collecting the copper. 



Experiment 1. Into some of this water, taken out of the stream above the 

 pits where the iron bars are placed, he poured a solution of an alkaline salt, 

 which raised a strong effervescence, and precipitated a large quantity of a dark- 

 brown substance. Which showed that the water contained a strong acid, with a 

 solution of the substance precipitated. 



Exp. 1. He put some aqua-fortis, or spirit of nitre, into water taken out of 

 the same place ; and observed, that the strong acid immediately destroyed the 

 blue colour. Whence he concluded that the substance, which was precipitated 

 by the alkali in the first experiment, was so perfectly dissolved by the acid spirit 

 in the 2d, as to transmit all the rays of light. 



Exp. 3. Some small iron nails put into this water, were in 4 minutes so 

 closely covered with some substance of a copper colour, that, with a magnifier 

 of \ inch focus, he could not discern the iron through it. In that time the nails 

 gained 4 grs. The water had the same effect on silver and tin, but not on 

 gold. The colour and increase of weight were owing to the adhesion of the 

 particles of the matter dissolved in the \\ater by an acid, that could not pene- 

 trate gold. 



