318 PROCEEDINGS OF THE ACADEMY OF [1880. 



From both these pair of analyses we have the ratio 



R: R : Si : H = 2 : 3 : 5 : 1 and R R : Si : H= 1 : 1 : i. The ratio 

 of bases to silica is 1:1, and for sesquioxides to protoxides. 



vr II 



S : R = 2 : 1. 



Philadelphite dried at 100 ^C appears to be a unisilieate, the 

 water not being basic. 



The formula ma}' perhaps be written 



£.: \\ O. \\ r 2. (K ^^-^"l -L 3. Q f S2 ^^ "(I,, J- 2 „., 

 •^- II ^-i II L \^>i' -i^J ^ o P K^^- 1 ^~ )j^ ^ 5 '^4 



The general symbol would be, 



11 VI 



R^jiVi, O20. Sio- 2 HjO. 

 The water will be regarded as water of cr3'stallization. Prof. 

 Cooke has shown the close chemical relation between the anhydrous 

 vermiculites and biotite. A like result is brought out by the fol- 

 lowing analysis of ignited Philadelphite. The analysis is a mean 

 of the two analyses of the anhydrous mineral which formed 

 analyses Nos. Ill and lY of the mineral dried at 100° C. 



100.12 



II VI 



Here R : ?^ : Si ^ 1 : 2 : 3, the ratio of a typical biotite. Anal- 

 gous as the anhydrous mineral is to biotite in its formula, it has 

 been shown that physically and optically the two minerals are 

 quite dissimilar, and it is not proven that they have any necessary 

 connection. It is by no means a hydrous biotite in the sense that 

 margarodite is a hydrous muscovite, in which case the characters, 

 optical and phj'sical, are identical. Such hydrous biotites occur 

 in several places in the vicinity of Philadelphia, in a partially 



