Primitive Crystals of Quart*, and the Sulphate of Barytcs. 237 



gether a mechanical operation dependent on the skilful use of the 

 goniometer ; and for this purpose doubtless brilliant and well de- 

 fined crystals have been selected. Having arrived at the admeasure- 

 ment of some one prominent angle, by means of the goniometer 

 in common use, it can scarcely be doubted that its value has been 

 assumed as a basis on which to calculate the rest. This, there is 

 reason to believe, was the process adopted by Hauy in regard to his 

 determination of the angles of the primitive crystal of the sulphate 

 of barytes, which he has given to seconds, and which the common 

 goniometer will not indicate. It must be obvious that the perfect 

 fidelity of this preliminary mechanical determination is most es- 

 sential. 



It is no trivial argument against the accuracy of this practice, 

 that those who are the most skilful in the use of the common 

 goniometer, have differed in the results obtained from the same 

 substances. But, even were it possible to depend upon the accuracy 

 of the instrument, and upon the skill of the hand using it, there 

 seems much reason for doubting whether the planes of such crys- 

 tals as are best adapted to its use can be relied on. I allude of 

 course to such as are of considerable dimension. These, when 

 submitted to the reflecting goniometer, rarely agree with each 

 other ; and the measurements they afford almost always differ very 

 considerably from those obtained upon small crystals ; which much 

 more frequently give similar results, but which are too minute, as 

 it seems to me, for the accurate use of the common goniometer. 



The difficulties already described as having attended my nume- 

 rous attempts to verify the admeasurements of Hauy in regard to 

 the primitive crystals of quartz and the sulphate of barytes, belong 

 not to the crystals of these two substances only. I have found 

 these difficulties to exist in different degrees in respect to more than 



Vol. iv. 2 h 



