IPS FILTERING STONE. 



possessing the same properties, and being applicable to the 

 same uses. 



There are ten in particular, that resemble it in want of 

 hardness, and size of grain, and that are capable of receiving 

 into their pores from eight to twenty-five hundredths of wa- 

 ter: such are, among others, those from Maillet quarries, 

 at St. Leu, and from the quarries at Vergelet, Gentilly, 

 St. Germain, Conflans, St.Honorine, and Boure, near Mont- 

 richard. 



The same author mentions, in the series of sand stones, 

 under the name of filtering sand stone, a piece from a stone 

 used as a filter, that absorbed a tenth of its weight of water ; 

 while the crystallized siliciferous carbonate of lime of Fon- 

 tainebleau did not absorb quite four thousandth parts ; and 

 True sand among the true sand stones, such as those used by paviors, 



stones absorb cu tlers, &c and even those in which the remains of organ- 

 much less wa- . . P 

 ter. ized bodies sometimes occur, there are none that admit so 



much. These circumstances lead to the supposition, that 



the specimen subjected to this trial by Mr. Brisson actually 



belonged to a filter of the same kind as those now generally 



used ; and that he gave it the name of sand stone, merely 



from the preconceived notion, that the property of filtering 



existed only in stones of this species. 



The properfil- We may conclude then, that the filtering stone employed 



carbmiat^of 21 ^ or domestic purposes at Paris is not a sand stone, but a 



lime, with -12 carbonate of lime, containing only 12 or 13 per cent of si- 



or 1-j.oisiex ^ eXf \ n sucn a s t ate of aggregation, as to leave pores suffix 



ciently open to admit water to run out of them gradually as 



they imbibe it : that it differs not only from the sand stones 



with siliceous cement, but likewise from the argillaceous 



sand stones, such as the grindstones of Geneva, Brives, &c. 



which in time imbibe a pretty considerable quantity of water, 



but let it pass through with much more difficulty: and that 



several of the quarries I have pointed out from Mr. Brisson 



may have calcareo-siliceous strata of the same nature, and 



possessing the same property. 



To remove all doubts on this head, it appears to me ne- 

 cessary to offer some remarks on the mode of determining 

 the specific gravity of substances with large pores. 



Errours in de- j t ma y a pp Car surprising, that I assign to the stone I have 

 tcimiiung the _ x * r o k ■ 



described 



