FILTERING STONE. \Q\ 



most modern works on mineralogy any mention of that kind 

 of stone, which is so mnch used at Paris for filtering water, 

 and the advantages of which have been confirmed by an ex- 

 perience of more than thirty years. 



This is a yellowish free stone, of a middle-sized grain, Common fil- 

 soft enough to be cut with a toothed saw, easily admit- p ar i s 

 ting its grain to be rubbed out by the fingers, and yield- 

 ing a fine powder, by rubbing two pieces against each 

 other. 



I found its specific gravity to be 2*322. A piece, weigh- Spec grav. 

 ing while dry 102*155 grammes, weighed 114*5, after it had 

 lain ten minutes in water, though it was carefully wiped ; 

 which gives an increase of 12*545 grammes, or very near an 

 eighth of its weight. 



A hundred decigrammes of this stone dissolved slowly in Effervesced 

 diluted nitric acid; and the carbonic acid gas evolved occa- w cus * 

 sioned a diminution of weight of 33*59, including the small 

 quantity of water, which it always carries off with it. 



The filtered solution left only 12*11 of siliceous earth. 



The lime precipitated by sulphate of pot-ash gave 139 of 

 sulphate of lime. 



Hence we may deduce the composition of this stone, con- 

 sisting of 



Carbonate of lime 87'89 i t3 component 



Silex 12*11 H pans. 



100 



I was desirous of knowing the place where this stone was p* a ce where 

 found in strata of sufficient extent, to supply the shops that found kept se- 

 work it up into filters for water; but from all my inquiries, 

 and what information I could get, it appeared, that the in- 

 ventor of these filtering stones, who has thus rendered a real 

 service to society, has thought proper to keep the knowledge 

 to himself. 



In consulting the description given by Mr. Brisson, how- Many similar 

 ever, in his Treatise on Specific Gravity, of the stones used stones u?ed ia 

 for building in Paris and its environs, from the collections 

 of Perronet and Wailly, several are found to exhibit the 

 same characters so completely, that we cannot doubt then- 

 possessing 



