360 ELEMENTS OF GENERAL SCIENCE 



does not begin to sour within twenty-four hours, it is time to 

 have it examined chemically to determine what preservative 

 it may contain. Refrigeration and pasteurization are the safest 

 methods of preserving milk, and these are effective for only 

 a day or two, since they retard but do not prevent the 

 development of bacteria. 



Various kinds of chemicals are used in the preservation of 

 meats. These chemicals are sometimes of a poisonous nature. 

 In most cases, however, preserved meats are injured more 

 by excessive quantities of relatively harmless preservatives. 

 An excessive amount of salt hardens the fiber of the meat 

 and destroys its normal taste. Salt, sugar, etc. are used to 

 cure meat because they take up water so rapidly that bacteria 

 are killed by the extraction of the water from the bacteria. 

 The older practice of smoking meat gave a coated surface 

 of creosote in which destructive organisms could not live. 



367. Food and filth. There is no inherent reason why small 

 particles of earth upon our food should be injurious to our 

 bodies, though they are always unattractive, but dirt of any 

 kind upon our food suggests the possibility of the presence 

 of living organisms that may prove injurious. It is more 

 important, however, to know that our food has been handled 

 only by persons who are free from disease germs than to know 

 that the food is not conspicuously dirty. Apples which are 

 shipped in barrels may have small particles of dirt adhering 

 to them and may not appear so attractive as wlien highly 

 polished, but if they are polished by the use of a soiled and 

 bacteria-laden cloth, they may thereafter be infinitely more 

 dangerous as food than when first removed from the shipping 

 barrel. Obviously, great care should be exercised to deter- 

 mine whether vegetables and other foods have been handled 

 in cleanly ways. A systematic examination of the vegetable 

 markets, with reports and discussions in class, will prove 

 instructive. Fortunately, thorough cooking will destroy bac- 

 teria, and unless one is reasonably sure that there is no 



