202 A HA T INFUSION. 



hay and water are in i state of comparative rest or equilib- 

 rium, but upon bringing them together action and reaction, 

 begin. First, the dust on the hay is wetted and soaked, 

 and any micro-organisms in it or adhering to the hay are set free, 

 and float in the water ; next, the water finds its way into the 

 stems and leaves of the hay, causing them to swell and resume 

 their original form. At the same time various soluble constitu- 

 ents of the daad grass, such as salts, sugars, and some nitrog- 

 enous substances, diffuse outward into the water, while from 

 such cells as have been crushed or broken open during drying 

 or handling, solid proteid or starchy substances may pass out and 

 mingle with the water. These simple physical reactions obvir 

 ously involve a disturbance of the chemical equilibrium of the 

 water. Originally able to support only a limited amount of life 

 (such as exists in drinking-waters), it is now a soil enriched 

 by what it has gained from the hay. The bacteria, extremely 

 sensitive to variations in their environment, and especially to 

 their food-supply, immediately proceed to multiply enormously, 

 so that a biological reaction follows closely on the heels of the 

 chemical change. But as a result of their metabolic activity the 

 bacteria set up extensive chemical changes, which in their turn 

 involve physical disturbances. For example, the dissolved oxy-. 

 gen with which the liquid was saturated soon disappears, so that 

 more oxygen must, therefore, diffuse into the liquid from the 

 atmosphere. Carbonic acid is generated in excess, and some 

 may pass outwards to the air. Also, as a result of the vital 

 activity of the micro-organisms the temperature of the infusion 

 may rise a fraction of a degree above that of the surround- 

 ing atmosphere. 



We are concerned, however, chiefly with the biological 

 results. In consequence of the exhaustion of the oxygen supply 

 in the lower parts of the liquid, many of the bacteria which 

 require abundant oxygen for their growth (aerobes) find 

 their way to the surface, where some pass into a kind o 

 resting stage (zooglcea) and form a scum or skin (mycoderm) on 

 the surface of the liquid. Others, for which free oxygen is not 

 necessary or to which it is even prejudicial (anaerobes), live and 

 thrive in the deeper parts of the beaker. But, meantime, an- 



