EARTHQUAKES AND VOLCANOES. 



force themselves up, have remained unchanged during the long 

 interval of 2000 years. 



As further examples of this permanency, Humboldt adduces 

 the example of a natural jet d'eau at Lillers, near Calais, which was 

 bored in 1126, and now, after a lapse of seven centuries, still sup- 

 plies the same quantity of water, which issues with the same force. 



50. If the reservoir from which water rises be at such a depth 

 that its temperature greatly exceeds the boiling point, the water 

 will be converted into vapour the moment it escapes from the place 

 of its confinement, just as was the case with the water shut up in 

 the generators of the steam apparatus projected by Perkins. In 

 that case, the steam will issue from the crevice of the earth 

 exactly as it would from the safety-valve of a high-pressure 

 steam-engine. 



51. The vapour of water is not however by any means the only 

 elastic fluid which forces its way to the surface from the interior 

 of the globe. Various gases are also ejected in enormous quan- 

 tities. The gas called carburetted hydrogen, which, evolved by 

 artificial processes, is now so universally used for the purposes of 

 illumination, issues in vast quantities from the interior of the 

 earth through fissures of greater or less magnitude, and thus pre- 

 sented by Nature herself, has actually been used for illumination 

 in China for more than ten centuries back. The artesian FIEE- 

 WELLS of China, at Ho-tsing are well-known. The gas has from 

 very ancient times been collected in tubes of bamboo, and being 

 thus rendered portable, has been used for illumination in the city 

 of Khiung-tscheu.* 



52. But of all the gaseous ejections from the interior, the most 

 frequent and abundant is carbonic acid. It is certain that at earlier 

 epochs in the history of our globe, thousands of centuries before 

 the appearance of man and of the other tribes of animals that now 

 inhabit it, when the fissures and crevices supplying free commu- 

 nication between the surface and the interior were far more 

 numerous and capacious than they are at present, and when the 

 temperature of the solid crust was much higher, this gas, mixed 

 with hot steam, issued from the interior in quantities infinitely 

 greater than at present, so as to give totally different qualities to 

 the atmosphere. These qualities, owing to the large proportion of 

 carbonic acid, and the great quantity of aqueous vapour always 

 suspended in the air, were eminently favourable to the production 

 of a vegetation exuberant to a degree of which there is now no 

 existing parallel. Hence arose those vast forests and other large 

 collections of vegetable matter, which, being fossilised in succeeding 



* Humboldt, "Central Asia," torn ii. 519530. 

 168 



