4.42 AGEICULTURAL AND INDUSTRIAL BACTEEIOLOGr 



in any way to consumers. Within recent years the most 

 common chemicals to be employed have been bleaching 

 powder, efficient because of its ability to release free chlor- 

 ine, and free chlorine gas itself. These usually disappear 

 relatively quickly by combining with various bases. As a 

 result there is a temporary but efficient disinfecting 

 action. 



Filtration. — Many cities purify their water (usually 

 after coagulation and sedimentation) by a process of filtra- 

 tion. It has been found by experience that if water is 

 allowed to settle through a bed of sand, bacteria are quite 

 largely eliminated. It must not be supposed that the open- 

 ings between the sand grains are so small as mechanically 

 to prevent the passage of the bacteria. Experience has 

 shown, however, that when 'water is being filtered in this 

 fashion, the grains of sand soon become coated with a gelat- 

 inous covering of microorganisms which oxidize the organic 

 materials of the water with which they come in contact. 

 These gelatin-coated sand grains have large powers of 

 adsorption, and the microorganisms coming in contact with 

 them usually cling to their surfaces. It is evident that sand 

 filters become more efficient after a moderate amount of use. 

 Eventually, of course, the surface coatings may be so heavy 

 as to obstruct the water and it becomes necessary to clean 

 them. 



Efforts are sometimes made in the home to purify water 

 by passing it through porcelain filters. When entirely free 

 from cracks and freshly cleaned, these are relatively 

 efficient, but like the sand grains, the pores in the filter are 

 in most cases large enough eventually to permit the passage 

 of microorganisms. They are, therefore, not usually advised 

 for permanent installations for the purification of water. 



Emergency purification of water can be quickly and 

 easily effected by boiling. All of the disease-producing 

 bacteria are destroyed at this temperature. 



