GEYSERS. 



605 



been portrayed, and are amongst the most wonderful phenomena of nature. 

 These will serve as a type of the other thermal springs, of which the districts 

 of the Yellowstone in North America afford perhaps the most extraordinary 

 instances. These are intermittent springs, and the water rises to a great 

 height, at intervals of about an hour and a half; and after many suc- 

 cessive attempts, or trials, as it were, the geyser shoots up to a great height 

 enveloped in steam. 



The cause of these 

 well-known phenomena 

 have been explained by 

 Bunsen, and it has al- 

 ready been referred to. 

 We know that at a cer- 

 tain air-pressure water 

 boils at 212 (Fahr.), 

 but on mountains at 

 less pressure it will boil 

 before that degree, be- 

 cause the air is rarefied. 

 So conversely, under 

 the ground, it may reach 



2 1 2 without 

 So the surface (warm) 

 water falls, and reaches 

 a high temperature be- 

 fore it is converted into 

 steam. When it is so 

 converted, the vapouris 

 formed 

 very 

 rapid- 

 ly, and 

 the ex- 

 pansive 

 for c e 

 is tre- 

 mendous Fig> 692 '~ Ge >' ser of the Yellowstone. 



shooting up the water and all the contents of the tube with terrific violence, 

 and with a beautiful effect. Pressure therefore alters the boiling point of 

 water. 



The mineral springs of Bath and many continental towns owe their 

 properties to the solvent power of water, which assimilates the mineral atoms 

 and gases. They arise just in the same way as the ordinary spring, the 

 taste and smell depending upon the soil and strata. Perhaps the oil wells 

 are the most curious phenomena of this kind. They are excavated upon 



