Mr. William Phillips on the OxydofTin. 349 



Haiiy; and 3'. 30". more than the latter. It may therefore be 

 presumed that the value of other angles connected with this, as 

 obtained by the reflecting goniometer, differ from those given by 

 Haiiy, both in the Traite and in the Tableau. I am perfectly aware 

 that It becomes me to speak with great deference on this subject. 

 I offer only the results of a mechanical attempt to ascertain the angles 

 of this substance, being incapable of verifying or of detecting their 

 fallacy by a recourse to calculation. 



The angle formed by the meeting of the planes P P. of the pri- 

 mitive crystal, fig. 18. PI. 15. is given by Haiiy as 67° 42' j by the 

 reflecting goniometer, I have uniformly obtained from clear re- 

 flections, an incidence of 67°. 50'. making a difference of 8 minutes. 



The incidences subjoined, are, for the most part, the result of 

 many perfect agreements of each, on different crystals. In no in- 

 stance has an average result been noticed. All are not to be relied 

 on with equal confidence. The plane forming the 9th modification 

 of the primitive octohedron is always so striated, and those of the 

 3d and 10th, are always so dull, that the incidences of those planes 

 with any other in the subsequent series can only be considered as 

 approximations. 



Incidence of P on P fig, 18. Pi. 15 ... 



P of either pyramid on its opposed plane over the apex 



1 on P fig. 21. PI. 16 



1 on 1 fig. 21 » > - . 



1 on 2 fig. 26 



2 on P fig. 26 - - - . 



2 on 2 of either pyramid over the intervening edge, fig. 27 121 



2 on its opposed plane 2, over the apex, fig. 27 



2 on 2 over the plane 1, fig. 27 



3 on 2 fig. 33 



4 on P fig, 39. PI. 17 - 



5 on 1 fig. 49 - - - - - 151». 35 



67=. 



50' 



112°. 



10' 



113°. 



25'? 



SO®. 





133°. 



32'. ZQ« 



150°. 



45'. 



12l». 



40'. 



92°. 



55'. 



87°. 



5' 



136°. 



35'. ? , 



123°. 



55' 



