180 Shepard's Treatise on Mineralogy. 



posed to have, (a cube,) I grant that it would establish its spe- 

 cific character, but the " negative evidence" seems to be as much 

 against Chatharnite as in its favor. 



Lincolnite. — In your measurement of Heulandite you suppose 

 M : e to correspond with the lateral angle of Lincolnite instead 

 of T : e, as shown by us in the last volume, p. 416. Taking 

 your new angles of Heulandite as correct, they give us 116° 17' 

 for the angle T : e, or the prism of Lincolnite. This angle is 

 1° 5' nearer your measurement of Lincolnite than that deduced 

 on the page referred to. 



Lederite. — In 3?"onr original description of Lederite, (this Jour- 

 nal, Vol. XXXIX, p. 359,) you state that the limits of variation in 

 the observed angles "were generally within 40'." We find on 

 comparing the angles with one another, reasons however for as- 

 suming much larger limits ; for example, if the angles P : a and 

 M : a (see the volume just referred to) are correct, P : M is ne- 

 cessarily 2° too large, as results from the fact that the three an- 

 gles of a triangle equal 180°, so also if P : c and M : c are cor- 

 rect, P : M is 1° 10' too much ; and if M : & is correct, M : M is 

 2° out of the way. There is therefore some reason for the al- 

 lowances that are made in Vol. xlvi of this Journal. 



Mr. Dana has handed me some new measurements with the 

 reflective goniometer of the so-called Lederite, made on a small 

 and bright crystal, put into his hands by myself, (from Ham- 

 mond,) in order if possible to settle the claims of this mineral. 

 The crystal is nearly identical in form with that figured by your- 

 self, except that the smaller planes are too minute for measure- 

 ment. He obtained, as he informs me, the following angles for 

 M : M, (r : r of the figure in Vol. xlvi, p. 36,) which are the 

 results of the first five trials, and were obtained without previ- 

 ously acquainting himself with the particular angle of sphene, to 

 which he supposed them to correspond. They are as follows: 

 n3°26'; 113° 14'; 113° 12'; 113° 28'; 113° 36'. 



The mean of these angles gives 113° 23 J' for r : r (M : M) in- 

 stead of 1 12° 10' as stated by you. Now r:r\s given by Phillips 

 (in which it is called d' on d') at 1 13° 24', and by Mohs at 1 13° 27' ; 

 a coincidence leaving little doubt of the identity with sphene. 

 If the plane a in your figure is identical with Phillips' e^, as 

 seems probable, your angle a : a is within 40' of the measure- 

 ment of Phillips. Yours, &c. B. S. Jr. 



Yale College Laboratory, November 12, 1844. 



