1823.] Dr. Traill on some Thermomagnetic Experimenis, 449 



Felspar and cleavelandite, it is remarkable, agree in their 

 earthy ingredients, which exist very nearly in the same propor- 

 tions in both substances ; but they differ in this, that the 13 or 

 14 per cent, of potash in felspar is substituted by about 10 per 

 cent, of soda in the cleavelandite, which, moreover, is not so 

 hard as felspar. These minerals possess natural joints parallel 

 to the planes of the doubly oblique prisms, which are considered 

 to be the primary forms of the two minerals ; but these forms 

 differ so completely in the measurements of all their angles, that 

 there is no hazard of mistaking the one for the other, after sub- 

 mitting them to the reflective goniometer. This, in consequence 

 of M. Levy's paper, we have done, separately, and with care, 

 and we find that in the Mount Sorrel rock, felspar and cleave- 

 landite are intermixed ; but it is impossible for us even to guess 

 their proportions as ingredients, since for the most part it is 

 difficult, frequently impossible, to separate them by the eye. 

 It may, however, be observed, that the felspar is frequently 

 translucent or transparent, and often reddish ; the cleavelandite, 

 white or yellowish-white, and nearly opaque, or various shades 

 of red, and that the very red veins traversing the rock here and 

 there, are chiefly of this mineral. Abundance of coinciding 

 measurements on fragments of both substances satisfy us of 

 their aggregation in this rock. 



I have since sought for the cleavelandite in other rocks, and 

 have found it, as well as felspar, in a beautiful porphyry from 

 Glen Tilt ; the specimen was obligingly presented to me some 

 years ago by Dr. Mac Culloch. In this specimen it is both 

 transpareat and colourless, and red and opaque. I have also 

 detected it in a porphyritic granite from Carnbrae in Cornwall : 

 in this specimen it is translucent and colourless, and white and 

 opaque, and felspar is more abundant in it. In the granite of 

 Shap, in Westmoreland, there is an intermixture of a whitish or 

 yellowish-white substance, of which some very minute and dull 

 fragments have afforded measurements within one degree of 

 those of the cleavelandite ; and I do not hesitate to believe that 

 better specimens would prove it to be that mineral. 



Article XIII. 



On some Thermomagnetic Experiments. By Dr. T. S. Traill. 



(To the Editor of the Annals of Philosophy ,) 



DEAR SIR, Liverpool Royctl Insiltution, Nov. 21,1 823. 



Having been lately engaged in some thermomagnetic expe 

 riments, I have met "with results which none of the papers 

 New Series, vol. vi. 2 g 



