HISTORY OF MINEEALOGT. 77 



celebrated "Werner had published a Systema Mineralogicum, 

 in which he had formally spoken of replacement by a plane, 

 an edge, and a point, as methods in which the forms of 

 crystals are modified. and often replaced. The wider appli- 

 cation of the plan was, however, due to Rome de 1'Isle, who 

 in his turn was eclipsed by the celebrated Haiiy ; who by the 

 variety, extent, and importance of his researches, during a 

 long life, exclusively devoted to mineralogical inquiries, may 

 be regarded as the founder of the school of modern crystal- 

 lography ; those who followed him having taken his views 

 either wholly or in part as their basis. He was the first 

 who successfully investigated the mathematical structure of 

 crystals, taking up the subject where it had been left by 

 Rome de 1'Isle. He determined the primary form of every 

 mineral, and showed how the secondary forms were derived 

 from it by simple laws of decrement. The knowledge 

 of these primary forms enabled him to arrange minerals 

 with more precision than had been done before him. He 

 defined a mineral species to be a substance compounded 

 of the same constituents, united in the same proportions, 

 and possessed of the same crystalline form. The chief 

 defects of the system of Haiiy, as contrasted with the pre- 

 sent state of the science, consist in the method adopted by 

 him of imposing a separate name on every secondary crystal, 

 and considering it as existing by itself. This multiplicity 

 of names renders it excessively difficult, it might be said 

 impossible, to remember all his secondary forms when they 

 are very numerous, as happens with respect to calcareous 

 spar, sulphate of barytes, iron pyrites, &c. ; and the perusal 

 of his book is thus rendered so irksome, that it can hardly 

 be undertaken without some specific object. But the grand 

 defect of his system consists in the inaccuracy of his measure- 

 ments, which were made only by the common goniometer, an 

 instrument not susceptible of giving the angle within half a 

 degree of the truth. After the invention of the reflective 

 goniometer by Dr. Wollaston, which is capable of measuring 

 angles within one minute, the angles of all crystalline bodies 

 were again examined by other mineralogists; and it was found 

 that those assumed by Haiiy were very seldom the true ones, 

 differing from the real measurement frequently by several 

 degrees. This general inaccuracy has rendered the measure- 



