SPONGES. 279 



isms, even molluscous shells or their impressions, 

 numerous infusoria, and star-like microscopic ob- 

 jects, which have been taken for fossil animalculse, 

 and termed Xanthidia, but which are probably the 

 spicula of fossil sponges. 



The colour of flints, agates, etc., is owing to 

 organic matter, and is consequently destroyed by 

 heat. When calcined and ground to powder, flints 

 are used to manufacture the finer sorts of pottery, 

 and which is termed flint-glass. Before the inven- 

 tion of percussion-caps, gun-flints were in general 

 use. It is a curious fact that sponges, one of the 

 softest of animal structures, should have contributed 

 so much to form one of the hardest of mineral 

 substances, and that men have made war and 

 slaughtered many thousands of their fellow-creatures 

 by means of sponges and infusoria ! 



Flints also form an excellent building material, 

 because they give a firm hold to the mortar, and 

 resist every vicissitude of weather. The counties 

 of Kent, Essex, Suffolk, Norfolk, etc., afford ex- 

 amples of many substantial constructions in flint 

 masonry. 



The uses of agates, for brooches, rings, seals, 

 etc., are too well known to need mention here. 



To return now to the toilet sponge, which con- 

 stitutes such an important article of commerce, and 

 about which I will add a few practical details. 



