MATERIALS EXISTING IN WATER. 37 



iron vessel, aud allows the steam to pass through pipes kept 

 constantly cold, by which means the steam again assumes its 

 fluid form, as we observe when a cold plate is placed opposite 

 the steam issuing from a tea-kettle, the impurities of the 

 water remain behind in the boiler. The same operation of 

 distillation is constantly going on in nature ; at every tempera- 

 ture and in every country, from the ice-bound seas of the 

 north to the tropics, it has been found that water slowly 

 passes to the state of vapour. The water on the surface of 

 the earth, in passing into steam, leaves behind it the matters 

 which it held in solution in its liquid state, and when it 

 descends again to the earth in rain it is almost pure. 



35. The solid matters that we discover in spring- water are 

 the same that are found in the rocks of the country, and when 

 we recollect that it contains carbonic acid, it is easy to under- 

 stand how lime, and other mineral substances not soluble in 

 pure water, may be dissolved by it from the rocks over which 

 it has passed in its course. Both the kind and quantity of 

 mineral matters which are found in the springs and rivers 

 of a country vary very much, and ai-e found to depend upon 

 the composition of the rocks or soils over which their waters 

 flow. The water of rivers, as might be expected, contains 

 less matter in solution than that of springs, which, penetrat- 

 ing slowly through the earth, dissolve aud take up the ingre- 

 dients of the beds of rocks or sand through which they pass. 

 In limestone districts we find the springs containing a large 

 amount of lime, while again where they issue from the granite 

 rock they are found to contain a mere trace of that substance, 

 and to be comparatively free from impurities.* Some waters 

 also contain a considerable amount of vegetable matter; thus 

 in the water of the river Lys in Belgium, I have found so much 

 as 2-86 per cent, of organic matters in the gallon. The quan- 

 tity of mineral matters dissolved in any water must evidently 



* Water of a stream near Rostrevor, County Down, an imperial grt. 



gallon contained of solid matter 10 



Water of a stream supplying a flax-pool at Moneyrea, Co. Down 10 



of a well at Shannon Grove, Co. Down 11 



of a pump do. do. do 28 



of a well at Stranmillis, near Belfast 60 



do. Irish Street, Downpatrick, Co. Down 140 



do. (St. Dillon's Well) do. 40 



do. Whitehouse, near Belfast 14^ 



do. Belfast 127 



