372 Scientific Intelligence — Hydrology. 



to wive rise to carbonic acid, ammonia, sulphuretted hydrogen, and 

 probably other gases. The ammonia generated, fortunately from the 

 same sources as the sulphuretted hydrogen, materially modifies its 

 influences. The consequences of the varying pressure of the atmo- 

 sphere have been observed; and it is shewn, that the exhalations of 

 sewers, &c., are poured out in abundance from every outlet where the 

 barometric pressure is lowered. By collecting the moisture of a 

 crowded room, by means of cold glasses, and also dew in the open 

 air, it was found that one was thick, oily, and smelling of perspira- 

 tion, capable of decomposition and production of animalcules and 

 confervffi, — but the dew beautifully clear and limpid. Large quan- 

 tities of rain-water have frequently been collected and examined by 

 Dr Smith ; and he says, I am now satisfied that dust even comes 

 down with the purest rain, and that is simply coal-ashes. No doubt 

 this accounts for the quantity of sulphites and chlorides in the I'ain, 

 and for the soot, which are the chief ingredients. The rain is also 

 often alkaline, — arising, probably, from the ammonia of burnt coal, 

 which is no doubt a valuable agent for neutralizing the sulphuric 

 acid so often formed. The rain-water of Manchester is about 2\° of 

 hardness, — hardei-, in fact, than the water from the neighbouring 

 hills, which the town intends to use. This can arise only from the 

 ingredients obtained in the town atmosphere ; but the most curious 

 point is the fact, that organic matter is never absent, although the 

 rain continues for whole days. The state of the air is closely con- 

 nected with that of the water ; what the air contains the water may 

 absorb, — what the water has dissolved or absorbed it may give out 

 to the air. The enormous quantity of impure matter, filtering from 

 all parts of a large town into its many natural and artificial outlets, 

 does at first view present us with a terrible picture of our under- 

 ground sources of water; but, when we examine the soil of a town, 

 we do not find the state of matters to present that exaggerated 

 character which we might suppose. The sand at the Chelsea Water- 

 works contains only 1 -43 per cent, of organic matter, after being used 

 for weeks. In 1827, Liebig found nitrates in twelve wells in Gies- 

 sen, but none in wells two or three hundred yards from the town. 

 Dr Smith has examined thirty wells in jNlanchester, and he finds 

 nitrates in them all. Many contained a surprising quantity, and 

 were very nauseous. The examination of various wells in the me- 

 tropolis shewed the constant fonnation of nitric acid; and, in many 

 wells, an enormous quantity was detected. It was discovered that 

 all organic matter, in fihating through the soil, was very rapidly 

 oxidized. The presence of the nitrates in the London water pi'e- 

 vents the formation of any vegetable matter; no vegetation can be 

 detected, even by a microscope, after a long period. The Thames 

 water has been examined, from near its source to the metropolis, and 

 an increasing amount of impurity detected. In the summary to this 

 report, Dr Smith states, that the pollution of air in crowded rooms is 



