35o POPULAR SCIENCE MONTHLY 



contain a considerable amount of these bicarbonates and soft- water 

 lakes have little or none. In hard-water lakes it is found that during 

 the growing season, when algse are active, the upper water contains no 

 free carbon dioxide, but is, on the contrary, alkaline, when tested with 

 phenophthalein as an indicator. This alkalinity conies from the fact 

 that one molecule of carbon dioxide has been withdrawn from part of 

 the bicarbonates, converting them into carbonates. It appears that the 

 algae are able to effect this reduction and that they can obtain their 

 supply of carbon from the carbon dioxide of the bicarbonates dissolved 

 in the water. This fact introduces a wholly new feature into the story 

 of the food supply of the plants. It provides a chemical carrier for 

 the carbon dioxide which may carry this gas somewhat as the hemo- 

 globin carries oxygen in the blood. All carbon dioxide set free in this 

 alkaline water as the result of decomposition or other processes, will be 

 taken up immediately by the carbonates. Thus if plants are not at 

 hand to utilize the carbon dioxide at once, it is not lost but kept until 

 it is needed. So in the night, the lake is able to retain all the carbon 

 dioxide set free and which the plants do not use at that time. 



Such alkaline water has also a great advantage in absorbing carbon 

 dioxide from the air. It presents for absorption, not merely the rela- 

 tively weak and slow powers of the water for dissolving the gas, but the 

 eager and vigorous powers of chemical affinity. And until these alka- 

 line carbonates are saturated, no free carbon dioxide will appear in the 

 water to diminish the rapidity of absorption from the air. Thus hard- 

 water lakes have an advantage over soft-water lakes in the matter of 

 securing plant food, and in fact the population of soft-water lakes 

 is smaller than that of lakes of the other type. 



It is worth while to devote a few words to gaseous products of 

 decomposition other than carbon dioxide. So long as the bottom water 

 contains an abundance of oxygen no other gas than carbon dioxide is 

 produced in appreciable quantities. But as the oxygen becomes 

 greatly reduced or wholly disappears, decomposition continues in new 

 forms and under these conditions of anaerobic fermentation other gases 

 may be developed in considerable amounts. It is apparently true that 

 earbon monoxide may be present in the lower water of lakes in appre- 

 ciable quantity, and it is certain that marsh gas is developed in large 

 volumes in lakes where the amount of fermentable material is large 

 and where the oxygen disappears from the lower water early in the 

 season. These gases first appear near the bottom, where decomposition 

 is going on most actively and where the oxygen first disappears. In 

 many lakes they are found only in small quantities and close to the 

 bottom, but in proportion as the amount of decomposable matter in- 

 creases, they are found at considerable distances from the bottom, and 

 in certain lakes all of the water below the thermocline may contain 



