132 WATERING HORSES. 



is poured backwards and forwards from one vessel into 

 another, it will absorb a certain proportion of the gases 

 which are in the atmosphere. Thus, a gallon of freshly fallen 

 rain water will generally contain about 4 cubic inches of 

 nitrogen, 2 cubic inches of oxygen, and I cubic inch of 

 carbonic acid gas (Bloxam). We here see that oxygen is 

 more soluble than nitrogen in water ; because air contains 

 about four volumes of nitrogen to only one of oxygen. The 

 absorbent power of water for gases diminishes in propor- 

 tion as its temperature is raised, as we may prove by 

 bringing into a warm room a glass jug full of fresh rain 

 water, in which case bubbles of gas which have separated 

 themselves from the water, will be seen to quickly form on 

 the inside of the jug. When stagnant rain water is exposed 

 to the rays of the sun, it will, on becoming heated, lose 

 a certain percentage of its dissolved gases, and will be far 

 less capable of recovering this loss than running water, on 

 account of the comparatively small surface it presents to the 

 atmosphere. Hence we find that rapidly running shallow 

 streams, especially if their course is a good deal broken up 

 by inequalities in their bed, are more highly aerated than 

 stagnant collections of rain water. Under the ordinary 

 pressure of the atmosphere, one volume of water will dis- 

 solve about one volume of carbonic acid gas. With a 

 pressure equal to that of two atmospheres, it will dissolve 

 two volumes ; with a pressure of three atmospheres, three 

 volumes ; and so on. Spring water, when below the surface 

 of the ground, often becomes charged with carbonic acid gas 

 under pressure. When this pressure is removed by the water 

 rising to the surface, the extra carbonic acid escapes with 

 more or less effervescence. This gas imparts to the water 

 a bright appearance and pleasant taste, which are in marked 

 contrast with the dull look and insipid flavour of soft water. 

 The gases contained in naturally aerated water promote 



