254* 0N THE PROCESSES FOR CLAXIFYING LIQUIDS. 



of a fuper-abundance of air with which they are fometimcs 

 impregnated, and gives them that lightnefs or brifknefs, which, 

 is found in the waters of the Seine in a greater degree than in 

 any other known ftrearn. Tho proof of this is, that by re- 

 peated filtraiions we may render water flat, heavy and un- 

 wholesome. 



Thus when the Specific gravity of the water of the Seine « 

 required to be determined, it is advifeable to take it from the 

 river at the time when it is limpid, or leave it to clear by 

 fubfidence, and not to take that in preference which has been 

 filtered ; for though this operation renders waters clearer, it 

 changes them remarkably, by depriving them, as has been 

 obferved, of the air which they contain in fuper-abundance. 



I know a perfon whofe palate was fo accuftomed to water, 

 that he could diftinguim by the tatfe water filtered through 

 Sand, from that which had not been fo filtered. The latter 

 appeared to him more fapid and lighter ; which proves, no 

 doubt, the privation of this air, a fact which may alio be very 

 eaiily afcertained under the receiver of the air-pump. 

 The filter can- Certain individuals, who are intercfted to maintain the con- 

 water irefh. trary to what is here aiTerted, have affirmed, that if water. 

 were continually forced to pais through 18 feet of fund of 

 gravel upwards, it would be purified not only of heterogene- 

 ous matters, but completely purified that is to fay of its falts; 

 This prejudice was fueh, that in order to fupport the notion 

 the following reafoning was offered ; 



If thefe filters be fufficient to deprive water of its air, why. 

 fhould not the fame operation be equally proper to deprive it 

 of the falts with which it is charged ; but no attention was 

 paid to the circumftance that thefe falts held in folution in the 

 water, being fpecifically heavier, pafs along with it through 

 the fmalleft apertures, whereas air being fpecifically lighter, 

 and exifling in a different fiate from that of the falts, is eaiily 

 Separated. And this procefs of application was propofed to 

 be applied to fea water in order to render it potable. The 

 operation has even been announced to the government as nevy 

 and ingenious. It confifled in a filtration effected by force 

 in a direction contrary to that of gravity. 



But the union of fajine matters with water has not -a purely 

 mechanical divifion. They are not interpofed as fome philo- 

 sophers have pretended, but perfectly diffolved in water, and 

 3 poflefa 



