Crystals. 121 



objection to it is that it is impossible to employ it in the case of very 

 small crystals, whilst the reflecting goniometer may be used to measure 

 accurately the angles of crystals only ,' ; ,th of an inch in size. 



Rome Delisle, as the result of his researches, came to the conclusion 

 that the primitive forms of all known substances were only six in number, 

 namely : 



i. The cube. 



2. The regular octahedron. 



3. The regular tetrahedron. 



4. The rhombohedron. 



5. The octahedron with a rhombic base. 



6. The double six-sided pyramid. 



These were announced in his treatise on Crystallography published 

 in 1783, in which he figures no less than 500 distinct forms of crystals. 



The weak point of his theory was the fact that the whole series of 

 forms of any one substance could be derived not only from the 

 primitive form, but from almost any of the series, thus rendering it 

 impossible to lay d nvn an exact rule as to which of these was to be 

 regarded as the true primitive form. He was guided in his choice by 

 the largeness of development and frequency of occurrence of particular 

 faces and the simplicity of the figure they formed. Thus he chose both 

 cube and regular octahedron, although, as we now know, these forms 

 really belong to one and the same series and may be derived the one 

 from the other. Many of his contemporaries doubted not only his 

 choice of primitive forms but the very existence of the series, and 

 Buffon's objections, as set forth in his " Natural History of Minerals" 

 published ten years later (1783), bore testimony to the difficulty of the 

 important step taken by Rome Delisle. It was far from being obvious 

 that all the crystalline forms of a mineral belong to one series. 



As early as 1773, Bergman, a celebrated Swedish chemist, shewed 

 in his writings that he recognized the importance of cleavage, and by it 

 he tried to explain the relationship of the various forms assumed by 

 the same mineral, which had so interested and puzzled Delisle, who, 

 however, assigned little or no importance to cleavage, speaking, as he 

 does in the preface to his treatise mentioned above, most contempt- 



