120 IGERNA B. J. SOLLAS. 



part of the lateral teetli. This last point may be confirmed 

 by performing the solution in the cold, when all the hard 

 parts are left in their natural relative positions. Prolonged 

 boiling with nitro-hydrochloric acid or hydrochloric acid and 

 potassium chlorate in an open test-tube failed completely to 

 remove the dark red-brown colour of the cusps; to accomplish 

 this the teeth must be placed with one of these solvents in a 

 sealed glass tube and heated in a water bath for some days. 

 After this treatment some of the cusps become complete!}'' 

 freed from their iron content and appear perfectly colourless 

 and transparent; others, however, after digestion for an 

 entire week retain some of their red-brown colour. 



The specific gravity of the teetli thus prepared was 

 ascertained by means of a diffusion column,^ in which the 

 majority were found to float in a dense zone at a level corre- 

 sponding with a specific gravity of r98, though numerous 

 examples ranged on each side of this to 1*87 on the one 

 hand and 2'08 on the other. This variation in specific 

 gravity corresponds with a difference in the degree of hydra- 

 tion of the silica : experiments made on colloid silica show 

 that when this is prepared from water glass it possesses a 

 specific gravity of I'SQ and contains 16*3 per cent, of water; 

 when obtained from silicon fluoride, its specific gravity is 

 1'98, and the water amounts to 9'85 per cent.; while sponge 

 spicules with a specific gravity of 2*04 contain 7 per cent, of 

 water. Thus it would appear that the water in the siliceous 

 basis of the Patella teeth varies in amount from 7 to IG per 

 cent, with a mean of 9"85 per cent. 



It is interesting to observe in this connection that the 

 association of water and silica in the silica hydrates occurs 

 without any change of volume in either of the constituents, 

 and thus the proportion of water in the hydrate can be 

 directly calculated from the specific gravity. In the follow- 

 ing table the results of such a calculation are given for a 



' Tor an account of tin's method sec Sollas, W. J., " Physical Characters 

 of Calcareous and Siliceous Sponge Spicules," 'Proc. Roy. Soc. Dublin,' 

 vol. iv, p. 378, 1885. 



