1864.] ADDRESS BY SIR CHARLES LYELL. 393 



nine feet in diameter, and no less than 140 feet in height. All this 

 matter is now quietly conveyed by a stream of limpid water, in an 

 invisible form, to the Avon, and by the Avon to the sea ; but if, 

 instead of being thus removed, it were deposited around the orifice 

 of eruption, like the siliceous layers which encrust the circular basin 

 of an Icelandic geyser, we should soon see a considerable cone built 

 up, with a crater in the middle ; and if the action of the spring 

 were intermittent, so that ten or twenty years should elapse be- 

 tween the periods when solid matter was emitted, or (say) an in- 

 terval of three centuries, as in the case of Vesuvius between 1306 

 and 1631, the discharge would be on so grand a scale as to afford 

 no mean object of comparison with the intermittent outpourings of 

 a volcano. 



Dr. Daubeny, after devoting a month to the analysis of the Bath 

 waters inl833, ascertained that the daily evolution of nitrogen gas 

 amounted to no less than 250 cubic feet in vol i me. This gas, he 

 remarks, is not only characteristic of hot-springs, but is largely 

 disengaged from volcanic craters during eruptions. In both cases 

 he suggests that the nitrogen may be derived from atmospheric 

 air, which is always dissolved in rain-water, and which, when this 

 water penetrates the earth's crust, must be carried down to great 

 depths, so as to reach the heated interior. When there, it may be 

 subjected to deoxidating processes, so that the nitrogen, being left 

 in a free state, may be driven upwards by the expansive force of 

 heat and steam, or by hydrostatic pressure. This theory has been 

 very generally adopted, as best accounting for the constant disen- 

 gagement of large bodies of nitrogen, even where the rocks through 

 which the spring rises are crystalline and unfossiliferous. It will, 

 however, of course be admitted, as Professor Bischoff has pointed 

 out, that in some places organic matter has supplied a large part of 

 the nitroiren evolved. 



Carbonic-acid gas is another of the volatilized substances dis- 

 charged by the Bath waters. Dr. Gustav Bischoff, in the new 

 edition of his valuable work on chemical and physical geology, 

 when speaking of the exhalations of this gas, remarks that they 

 are of universal occurrence, and that they orginate at great depths, 

 becoming more abundant the deeper we penetrate. He also 

 observes that, when the silicates which enter so largely into the 

 composition of the oldest rocks are percolated by this gas, they 

 must be C(»ntinually decomposed, and the carbonates formed by 

 the new combinations thence arising must often augment the 



