each substance may be most readily attained. Concluding that the same difficulties are felt by others, I shall add some remarks on that subject, in regard to such of the substances as I have been able to cleave with regularity, presuming that it may tend to render the way more easy for those who may desire to attain the same object. It must be obvious that very different means have been resorted to; for no one will imagine the same to be applicable to the sulphate of barytes and the sapphire: one of them soft, and yielding to mechanical division with the utmost ease; the other, the hardest of all the earthy minerals, and splitting only by the application of great force, and even then, not without much difficulty and perseverance in more than one direction. Still further remarks will be needful in regard to one of the substances, the sulphate of lead, since the mechanical division it affords has necessarily led me to differ from the Abbé Haüy and the Count de Bournon, as to the form of its primitive crystal.
The following pages would not have been offered to the notice of the Geological Society, but for such reasons as belong to the importance of determining with precision not only the forms, but the measurements of the angles of primitive crystals. If it should be thought that it is assuming too much, to differ from authors so distinguished as the Abbé Haüy and the Count de Bournon, I beg to offer the same apology as was offered by the latter for differing from the former in the same respects. “The attainment of truth is the great object that every man 'ought to propose to himself, who has any pretension to science.”[1]
Such of the figures in the accompanying drawing [2] as suited my purpose were copied from those of Haüy, and when compelled to alter the form, the letter by which he designates each plane, has been studiously retained for the more ready reference to his works.
