CHAPTEE XYII. 



A HAY INFUSION. 



If a wisp of liaj is put into a beaker of water and the mix- 

 ture allowed to stand in a warm place there is soon formed what 

 is known as a hay infusion. Microscopical examination of a 

 drop of the liquid at the end of the first hour or two reveals 

 little or nothing, and if the beaker be held up to the liglit the 

 liquid appears clear and bright. But after some hours a marked 

 change is found to have taken place. The liquid, originally 

 clear, has become cloudj, and a drop of it examined microscop- 

 ically wall be found to be swarming with bacteria. A day or 

 two later, the cloudiness meanwhile increasing, the microscope 

 generally reveals not only swarms of bacteria, but also numerous 

 infusoria. At the same time the color of the liquid has deep- 

 ened, it begins to appear turbid, a scum forms on the surface, 

 and the odor of hay, which was present at the outset, is replacec* 

 by the less agreeable odors of putrefaction. The simple ex- 

 periment of bringing together hay and water has, in fact, set in 

 motion a complicated series of physical, chemical, and biological 

 phenomena. 



The Composition of a Hay Infusion. A hay infusion consist? 

 of two principal constituents, hay and water. But neither i)\ 

 these is chemically pure. Hay is only dried grass which for 

 weeks, and even months, was exposed in the field to wind and 

 dust. Covered with the latter — often the pulverized mud of 

 roads and roadside pools — hay is richly laden with dried bacteria 

 and other micro-organisms; while water, such as is ordinai-ily 

 drawn from a tap, frequently contains not only an abundance of 

 free oxygen and various salts in solution, but also numerous bac- 

 teria, infusoria, algae, diatoms, and other micro-orgamsnis iu 

 suspension. In the making of a hay-infusion, therefore, numer- 

 ous factors co-operate, and a series of complicated reactions 



follow one another in rapid succession. At the start both 



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