Crystalline Forms of the Hayiorite. 45 



crystals of the first have only borrowed the forms of the other; 

 and therefore the above suggestion must be abandoned. The 

 only substance between the angles of which and those of 

 Hay tori te there seems to be a great analogy is Hamboldtite. 

 First, by inverting the drawings of Mr. Phillips, the similitude 

 of the forms of Haytorite with that of Humboldtite repre- 

 sented by Mr. Phillips, (see the Elementary Introduction 

 to Mineralogy, p. 3S0,) or the one I have given in the number 

 of the Annals of Philosophy for February 1823, becomes 

 apparent The planes P,^', d,. h, k, i, n, v of Haytorite cor- 

 responding to the planes of Humboldtite, Mr. Phillips has de- 

 signated in the work above referred to, by h, m, a.^,J', e.^, e^a^g^: 

 the incidences of these different planes measured by Mr. Phil- 

 lips are, 



The agreement between these several angles is not perfect : 

 but it must be observed that some doubt may still be enter- 

 tained with respect to the an^es of Humboldtite. Some of the 

 measurements of that substance given by Mr. Phillips, indi- 

 cate that the base of the primitive makes an angle less than 

 90° with the plane h ; whilst from the observations I took as 

 data, I found that angle equal to 91° 41' 30" *. There is not 



therefore 



* Professor Mohs and Mr, Haidinger consider Humboldtite as a variety 

 of Datholite ; and their opinion is supported by a great analogy between the 

 two substances. However, I think there can be no inconvenience to pre- 

 serve the distinction till some better measurements of Humboldtite can be 

 obtained, especially as it appears to me there are still some reasons to make 

 doubtful the propriety of the re-union. In order to refer the crystals of 

 Datholite and Humboldtite to the same primitive form, the incidences of 

 the planes of Humboldtite I have designated by e', and Mr. Phillips by 

 e*, should be the same as the incidence of the planes M of Datholite. The 

 first incidence 1 state in the paper inserted in the Annals of Philosophy for 

 February 1823, was one 1 could obtain most accurately, and I found it equal 

 to 102° 30': the second 1 found 103" 25'. Mr. Phillips in his elementary 

 work on Mineralogy gives the result of his own observations of these two 

 angles 102° 35', and 10.3" 40'. At my request he was so good as to take 

 again some measurements of Datholite, and found Mon M 103" 40', 10.'5"38', 

 103" 35'. The agreement in the amount of the difference of these two an- 

 gles, by two different observers, is perhaps sufficient to throw some doubt 

 upon the accuracy of the opinion founded on their equality. To a certain 



extent 



