THE MOLLUSCAN RADULA. 121 



small number of cases^^ tlie specific g'ravity of amorphous 

 silicon dioxide being taken as 2"21. 



SiOo, HoO, water'23 per. cent., sp. gr. 1-73 (8102)3, (HoO),, 

 water 16"6 per cent., sp. gr. 1*85. 



(SiOo).,, H3O, water 13 per cent., sp. gr. 1-9 ; (SiOo). H.O 

 water 9'1 per cent., sp. gr. 2"0. 



(SiOo)^, HjO, Avater 7 per cent., sp. gr. 2*04. 



Under the microscope the teeth which have been treated 

 as described above present a faint brown granular appearance 

 when seen by transmitted light and a bright milky white by 

 reflected light : in this behaviour, which results from the 

 abundant presence of minute pores, they resemble common 

 opal. Between crossed Nicols they exhibit faint but evident 

 double refraction with undulose extinction ; the same 

 character is sometimes presented by mineral opal, and is 

 attributed to internal stress, the existence of which in the 

 teeth is suggested by their liability to spring apart along 

 fractures parallel to their length. 



The refractive index of the siliceous residue of the teeth 

 was determined by Becke's method and found to be closely 

 approximate to r45 ; the fluids used were mixtures, one of 

 10 parts, the other 20 parts of Price's glycerine, to 1 part 

 of water. Taking the refractive indices of these mixtures to 

 be 1"449 and 1"454, the above result is obtained. 



Teeth from which the iron has not been completely ex- 

 tracted are distinguished by a relatively high specific gravity; 

 under the microscope the ferric hydrate or oxide presents a 

 blood-red colour by transmitted light and marked double 

 refraction. 



The ribbon of Patella, when soaked for some hours in 

 strong hydrofluoric acid, becomes colourless or nearly so 

 throughout, but the forms of the teeth are perfectly pre- 

 served. If it is next boiled in nitric acid it dissolves com- 

 pletely, as might be expected, since the siliceous matter has 

 already been removed by the hydrofluoric acid. On incinera- 

 tion the ribbon retains its form with a surprising completeness. 

 The ash proves to contain, in addition to silica already 



