120 The Ottawa Naturalist. 



in the year 1772. Rome Delisle,a pupil of Linnaeus, shewed that the various 



shapes possessed by crystals of the same substance, natural or artificial, 

 are all intimately related to each other. He formed a large collection of 

 natural crystals which he carefully studied and was particularly interested 

 by the fact that the same mineral often occurred in widely different 

 forms. His studies led him totheconclusionth.it the shape of every 

 crystal of the same substance is such as can be derived by a particular 

 process from a certain fundamental figure called the Primitive Form, 

 the shape and angles of which depend only on the nature of the subs, 

 tance itself. All the multitudinous forms which asubstance such as pyrite 

 (sulphide of iron) assumes,he found could be produced by replacing the 

 edges or the solid angles of the primitive form by single planes or groups 

 of planes, but always in such a manner that the total alteration is 

 similarly related to all parts of the primitive form which are geometri- 

 cally similar. 



Thus, as a simple example, by cutting off the angles of a cube it 

 may be converted into an octahedron. These planes of replacement were 

 regarded by him as being secondary and more or less accidental. 



Werner in his treatise " On the External Characters of Minerals " 

 had employed the terms Abstumpfnng^ truncation, '/jischarfiing ^= bevel- 

 ling, 'Luspitzung = acumination, in speaking of similar variations or 

 changes from the fundamental form of crystal, but it is thought that Delisle 

 did not know of this at the time he wrote. Delisle set to work to determine 

 the primitive forms of all substances, which work was greatly facilitated 

 by the invention at this time of the goniometer. This instrument 

 was invented by a Frenchman named Carangeau, who prepared the 

 clay-models used by Delisle to illustrate his theory. It was designed 

 for the measurement of solid angles, particularly those of crystals, and 

 was of the form known as the common or contact goniometer. 



A much more elaborate and accurate instrument for the same pur- 

 pose is the reflecting goniometer of Dr. Wollaston, devised by him in 1809, 

 of which several elaborate modifications are now employed by crystallo- 

 Hiaphers. Carangeau's goniometer consisted essentially of a graduated 

 arc and two moveable arms. Its form may be learned by referring to 

 the figures given in almost all text-books of mineralogy. The great 



