Tyndall On The Purity Of Water

l'Vom Havpcr's. Our readers will remomber the interest oxcited daring the pust year by a looture given by Professor Tynd.ill before tho Boyal Institution upon Dust and I)i in which he presentad soine gtartling as to the iinpurity oí' the atmosphere, and made soine important suggestions as to the method of improving the quality of the air we breathe. The Professor has lately delivered a Iceture npon the color of water, and the scattering of light in water and air, which will probably bo of equal practioal value with that ürst referrcd to. His subject was illustratcd, as before, by passing a ltam of light tlirough theliqnid in a darkenud room, by means of which the oxistence of tho minuto partióles of impurities can be readily deteoted. Lately engaged as one of the savans of the eclipse expedition, he embracod the opportunity to gathei samples of water in viu-iuus looalitiea iind al differeni di pths, and these weresecuredand sealed ap with the utmost care, so us to BiTOÏd the possibility of the introduotion of nny additional impurities. In the caso of all the specimens of water obtained within a few miles oí tno snore me uetmi oí iiul ievealed inore or less impurity ; and it ivas only in the indigo water of the sea (asdistingoished i'rom tho green) that he found a decided reduotion is the imount oí' foreign ingrediente. Ho assures os that th provalonco of' ono or tho other of these lints iu sea water is alwava expressive oí' a greater or leas aegree oí purity, es depending upon Biispended matter. Ho advises tho use of an experiment similar to that referred to tbr determining the purity of water of ny kind, ospeoially that used for drinMng, as, notwithstandiag a. oareful filtering through poroua and oven thvough a charcoul iilter, there niaybeleftin suspension matter dimos! too fine fov detoction by (lie mioroacope, and yet olearly iadioatea by the beain of light He stated in his loeture that tho purest water that ho was ahle to íind was obtained by meltinga block ofpure ice, bul that even this required extreme oaution to insure snocess. lie called attention to the remarkable purity of the water from the ohalk distriota of England, and remarked that but t'or the bardnew ot' the water, or the amouut of carbonate of lime held in solutiou, it vronld the most desirable for drinking and other pnrposes, - 1 1 ■ ; tiii, liowevcr, that at Canterbury and elsewhere the water is subjeoted to a speeial preparatioo that oauses adepoát oí' the lime. This is aocomplished by addine olear prepared lime-watei to the obalk-water, thereby oattsing'a preoipitate of oarbonate of lime to the botti the reservoir. By tliis means thfl percfiíi of this BftH in the water i ■■ ñsduce1 froin 17 to M, lea-ñrtga water oí' extraordinary beauty and juinty. f