12 



Corbeil (20 miles above Paris) Dissolved Oxygen — 9.32 CC per litre. 



Epinay (below all the sewers) " " = 1.05 " " 



Pont de Poissy (49 miles below Paris) . . " " = 6.12 " " 



Pont de Meulaii (58 miles below Paris). " " — 8.17 " " 



Mantes (68 miles below Paris) ' " =8.96 " 



Vernon (94 miles below Paris) " " — 10.40 " " 



These numbers are very easy to explain when we consider that the 

 decaying organic matter brought into the river by the sewage of Paris 

 consumes the dissolved oxygen, and is by this consumption of oxygen, 

 converted into other and comparatively harmless compounds, so that, at 

 a point 90 miles below the city and 70 miles below the sewer mouths, 

 the river regains its normal condition as far as this factor is concerned. 



(larbon di-oxide, or carbonic acid gas is much more soluble than 

 oxygen. Roughly we may say that water dissolves its own volume of 

 this gas. The only other gas which I shall mention is ammonia, and 

 the extreme solubility of this gas in water is well illustrated in the ex- 

 periment before you, in which the first portions of water entering the 

 large flask filled with ammonia gas dissolve the whole of the gas there- 

 by creating a vacuum into which a fountain plays — the red liquid (a 

 slightly acid solution of litmus in water) being constantly changed into 

 blue in the fountain jet, and thus bearing witness to the alkaline char- 

 acter of the ammonia. 



The solubility of gases in water becomes less as the temperature 

 rises. It is for this reason that water that has been boiled and allowed 

 to cool makes so flat and insipid a beverage. The atmospheric gases, 

 and particularly carbonic acid gas, have been expelled at the boiling 

 temperature, and the water requires artificial aeration before it can 

 become again a sparkling and palatable drink. Under increased pres- 

 sure a very much larger amount of gas can be held in solution. Effer- 

 vescing drinks like soda-water, ale and champagne are kept in strong 

 bottles with corks wired down. When the bottle is opened, and ordi- 

 nary atmospheric pressure applied to the surface of the liquid, the excess 

 of gas which could only be kept in solution by abnormal pressure 

 escapes, and gives the sparkling effervescence characteristic of these 

 beverages. 



Unlike gases, a rise in temperature is usually attended with a 



