LESSON VIII 



BACTERIA 



It is a matter of common observation that if certain moist 

 organic substances, such as meat, soup, milk, &c., are allowed 

 to stand at a moderate temperature for a few days — more or 

 fewer according as the weather is hot or cold — they "go 

 bad " or putrefy ; i.e. they acquire an offensive smell, a taste 

 which few are willing to ascertain by direct experiment, and 

 often a greatly altered appearance. 



One of the most convenient substances for studying the 

 phenomena of putrefaction is an infusion of hay, made by 

 pouring hot water on a handful of hay and straining the 

 resultant brown fluid through blotting paper. Pasteur's 

 solution may also be used, or mutton-broth well boiled 

 and filtered, or indeed almost any vegetable or animal 

 infusion. 



If some such fluid is placed in a glass vessel, covered with 

 a sheet of glass or paper to prevent the access of dust, the 

 naked-eye appearances of putrefaction will be found to 

 manifest themselves with great regularity. The fluid, at first 

 quite clear and Hmpid, becomes gradually dull and turbid. 

 The opacity increases and a scum forms on the surface : 

 at the same time the odour of putrefaction arises, and 



