500 Mr. R. Mallet on the 



The phenomena of geysers were for a long time supposed peculiar to 

 Iceland; and although they are now known to exist elsewhere, their 

 characteristics are nowhere better observable than in Iceland. 



The recurrence of their outbursts, their duration and intervals, were 

 very well described by Von Troil in his ' Letters on Iceland ' in 1772, and 

 have been further described by Sir George Mackenzie in 1810. 



Henderson had ascertained that stones, or other obstacles, thrown into 

 the geyser-tube influenced the interval between two outbursts generally 

 by increasing it, and gave rise to augmented violence in the outburst when 

 it came, the stones being projected back along with the water, and rising 

 much higher than the latter, as might have been predicted from dynamic 

 considerations. Sir John Herschel suggested an explanation of geyser- 

 phenomena, based upon modifications of the mechanism long previously 

 proposed to account for those of intermittent springs. His explanation, 

 though tenable, certainly does not apply to all observed cases, and is scarcely 

 likely to be the true one, because a much simpler mechanism has been 

 since pointed out ; and it may be taken as certain that, in explaining all 

 natural phenomena, the simplest is the true one. This was discovered 

 by Bunsen and Des Cloizeaux, who in 1846 examined the geysers of Ice- 

 land, and ascertained the fact that towards the bottom of the tube of the 

 Great Geyser, at a depth of 78 feet from the lip of the basin, a thermo- 

 meter immersed in the rising column of water rose to 266 Eahr., or to 

 more than 50 above the boiling-point of water, under atmospheric 

 pressure only; and these authors conclude that, as the flow of water which 

 replenishes the tube after an outburst causes the aqueous column gradually 

 to rise to the lip of the basin, the temperature of the water at the lowest 

 part of the column continues to rise ; and whether it receives its accession 

 of heat from the sides of the tube or from jets of superheated steam 

 issuing into it, no considerable volume of steam can be generated until 

 the boiling-point has been reached at the bottom of the column, as due 

 to its insistent pressure there, when a sudden and large outburst of 

 steam takes place, and the whole column of water is belched forth from 

 the tube, succeeded by the blowing-off of the pent-up steam which ex- 

 pelled it, and with steam evolved from the column of water as it rises, 

 until that falls back to atmospheric pressure. The curious facts ascer- 

 tained by Professor Donny, of Ghent, that water absolutely free from 

 combined air may be heated to even 275 Fahr. before it boils, and then 

 bursts into steam explosively, have been appealed to as auxiliary to the 

 phenomena, but seem unnecessary, even were it certain that the water of 

 geysers is absolutely air-free. Were it so, however, there can be little 

 doubt that the rise in the boiling-point of such water, under atmospheric 

 pressure, would also take place in the same water under a head of 78 feet, 

 or equal to more than two atmospheres, and thus would still further aug- 

 ment the temperature at the bottom of the tube, and further increase the 

 violence of the outburst. 



