1820.] Memoirs of the Liter ary Society of Manchester. 141 



ences respecting" the foreign substances present in the water 

 under examination. If it becomes milky with Hme water, he 

 infers carbonic acid or bicarbonate of hme ; if it is precipitated 

 •white by oxalate of ammonia, he infers the presence of a salt of 

 lime. Muriate of barytes indicates sulphuric acid, and nitrate of 

 silver muriatic acid. But to determine the quantities of the 

 bodies present in water is much more difficult. I doubt whether 

 Mr. Dalton's methods be susceptible of much precision. It is 

 very difficult to determine the exact point of neutralization in 

 very dilute solutions ; nor do I beheve that the exact quantities 

 of the hquids employed ctm be determined by measure. Weight 

 seems the only accurate method. Mr. Dalton expresses his 

 surprise at finding that lime, though supersaturated with carbonic 

 acid, still acts as an alkali on vegetable colours. A little consi- 

 deration will solve this difficulty. Suppose we dip into a solution 

 of bicarbonate of lime a bit of litmus paper previously reddened 

 by vinegar. The paper owes its red colour to the presence of 

 acetic acid. This acid is capable of displacing carbonic acid 

 from all bases, and uniting with these bases in its place. When 

 the paper is dipped into the liquid, the acetic acid leaves the 

 litmus to unite with the lime ; and if a sufficient quantity of lime 

 be present, the paper must resume its blue colour ; that is to 

 say, lime must always act as an alkali when it is combined with a 

 weaker acid than that which disguises the vegetable blue colour. 

 There is no difficulty then in seemg the reason of the fact which 

 surprised Mr. Dalton so much. 



V. Accoiud of the Floating Island in Denoent Lake, Keswick. 

 By Mr. Jonathan Otley. — The name of Floating Island is given 

 to a portion of earth, about six feet in thickness, which rises 

 occasionally in the south-east corner of the lake, not far from 

 Lowdore, generally about 150 yards from the shore. It is 

 attached by one side to the bottom, and sometimes extends to 

 the length of above an acre, and sometimes constitutes only a 

 few perches. Its rise is only at uncertain intervals. Sometimes 

 it appears in two successive seasons, and sometimes not during 

 an interval of seven or eight years. Sometimes it rises a foot 

 above the surface of the lake, and sometimes remains several feet 

 below that surface. It usually rises to the surface at the end of 

 a warm dry season. The bottom of this lake seems to consist of 

 an imperfect kind of peat moss ; and Mr. Otley suggests that 

 part of the bottom is occasionally buoyed up to the surface in 

 consequence of the great quantity of gas that is generated 

 within it. Mr. Otley and Mr. Dalton collected a quantity of 

 this gas in 1815. Mr. Dalton found it a mixtm-e of nearly equal 

 volumes of carburetted hydrogen and azotic gases with about 

 six per cent, of carbonic acid gas. No oxygen could be detected 

 in it. This absence of oxygen I think remarkable. I have found 

 common air always mixed with the specimens of carburQU^d 

 hydrogen gas collected from stagnant water-pools. 



\To be continued.) 



