284 



Mr. Levy on the 



work, by preparing the crystals, upon the purity of wliich I 

 could therefore depend. At his recommendation, Mr. Brande 

 has also allowed me to select some crystals from those in his 

 collection, preserved in the laboratory of the Royal Institution, 

 and at Apothecaries' Hall. 



With this help, I propose to employ some leisure hours to 

 the determination of as many crystallized substances as I shall 

 be able to procure. This paper, and some subsequent ones, 

 will contain the result of my researches. Besides the primi- 

 tive, I shall give one or two of the forms which most com- 

 monly occur. I measure the angles with a goniometer belong- 

 ing to Mr. Lowry, and which is divided to half a minute ; and 

 I besides use the principle of the repetition of angles, in order 

 to obtain a greater accuracy. At the suggestion of Dr. Wol- 

 laston, I call the solid, from which the secondary forms are 

 supposed to be derived, by the appellation of ■primitive, when 

 obtained by cleavage ; and by that of 'primary^ in the contrary 

 case, I designate the angles and edges of the primitive, by 

 the same letters as Haiiy ; and the secondary planes, by the 

 signs of the decrements from which they are supposed to result. 



I have begun with the salts of potash. 



Nitrate of Potash. 



Incidences. 



m on m 109° 50' 



bi 135 36 



gr' 160 42 



e' on gi 120 25 



Primitive form. — A right rhombic prism, the incidence of 

 the two lateral p'anes of which is 109° 50', and the ratio be- 

 tween one side of the base and the height nearly that of 

 1 to 0,48. 



