Mr. William Phillips on the Oxyd of Tin. 373 



that ah l k g and a h Imf are similar planes, each consisting of one 

 isoscele and two halves, of similar triangles. It follows therefore 

 that this made is composed of three sections of the primitive prism, 

 fig. 196, alternating with six halves, two and two, of the same 

 figure. 



It should be noticed that the angle / m r, fig. 200, to which there 

 are five others similar, is about 120° by the common goniometer ; 

 but as the edges of the made are very uneven, it cannot be relied 

 on for admeasurement. The angle Imf like which there are also 

 five others, nearly agrees, but is not, accurate for the same reason 

 with that oi aec^ fig. 193, which is the result of a close com- 

 bination of two similar isosceles triangles. 



It will be understood that fig. 208, PL 25, and the seven succeed- 

 ing figures, comprehending the series of that which is commonly 

 termed the dodecahedral made, (each being numbered with the figures 

 of the several modifications of which it shews the planes) are not in- 

 tended to represent dodecahedrons, as the macles themselves consist 

 only of what is visible in the respective drawings, or at most of only 

 one-third more, that is, of three or at most of only four sections of the 

 prism, fig. 196, PI. 24. Yet the apices of several of them are perfect, 

 as for instance, the plane on the summit of fig. 210, which is perfectly 

 defined, and which therefore indicates the regular combination of 

 six sections of that figure, unitedly exhibiting a decrease on the 

 apex by the plane of the fourth modification. As a corroborative 

 proof, however, that these macles, under the most favourable cir- 

 cumstances for perfect crystallization could never become perfect 

 dodecahedrons, it may be observed that several in the series which 

 exhibit those planes of the fourth modifications which give to fig. 

 210 the form of a short prism, give uniformly an incidence of 4 on 4 

 by the reflecting goniometer of 112". 10'. ; whereas if they were 



