28 



BACTERIOLOGY 



organisms passing through and being found even in the best 

 water-supplies, an arbitrary standard of reasonable purity being 

 laid down. It has been found by experience that water, say for 

 a town water-supply, is greatly improved as regards its bacterial 

 content by being stored for a varying period in reservoirs before 

 the filtration is carried out, a large number of bacteria dis- 

 appearing spontaneously in the process. Similarly ordinary 

 river-water becomes much purer by passage through a lake or 

 reservoir. 



Another very important method of destroying bacteria is by 

 the addition of chemical disinfectants. Thus 

 diseased discharges such as tuberculous sputum, 

 typhoid stools, etc., may be rendered non- 

 infective by the addition of 

 a sufficient amount of lysol, 

 or other suitable disinfectant, 

 which must be allowed to act 

 for a sufficient length of time 

 and in sufficient concentra- 

 tion to kill the organisms 

 present. Formalin in the 

 form of a spray is now ex- 

 tensivelyused in public health 

 work for the disinfection of 

 infected premises, etc. Many 

 substances are also used in 

 order to prevent the develop- 

 ment of organisms, and are 



FIG. 7.-a a Chamberland'3 Porous Filter, therefore known as anti- 

 mounted in metal tube with tap to septlCS, e.g. boracic or boric 

 fit the water supply for domestic use. acid, carbolic acid and many 

 ^.fflSSSSaSSiffi; of its relatives, salicylic ackf, 

 as toxins, serum, etc. iodine, formalin, and the like. 



Some of these are used for the 



dressing of wounds in order to prevent the occurrence of sepsis 

 or infection by organisms, and of course most of them also act 

 as disinfectants if organisms are already present. A very im- 

 portant present-day problem is the question of the addition 

 of such substances as boracic acid, salicylic acid, formalin, 

 etc., to articles of food, e.g. milk, jam, etc. The constant taking 

 of some of these into the human body may in some cases be 

 highly deleterious, and, moreover, may give rise to a false 

 security as regards the presence or absence of micro-organisms, 

 as some of them merely inhibit the growth of putrefactive 

 organisms, whilst the disease-producing bacteria may not only 

 not be destroyed, but may actually have a better opportunity 



