400 VETEEINAEY HYGIENE 



These examples are only given as illustrating the case in 

 point, viz. if thoroughness of disinfection is to be carried 

 out it must be done under competent direction. The super- 

 vising authority should know something of the vitality and 

 life history of the organism he is endeavouring to exter- 

 minate, he must know its favourite habitats, he must fully 

 realize its extreme minuteness, and that an undisinfected 

 patch, impossible to see with the unaided eye, may con- 

 tain sufficient of the poison to start the whole disease 

 afresh. 



We have dwelt on this question, as it is important the 

 public should understand when it is paying money to exter- 

 minate disease, that the destruction of the affected is only 

 the first step towards that end ; the next, and equally 

 important, is thorough disinfection. The latter is usually 

 left to lay control, with the result that it is imperfectly 

 performed in consequence of ignorance. It is just as neces- 

 sary for thorough disinfection as for accuracy in diagnosis 

 that the expert should be employed, otherwise compensa- 

 tion may be a waste of public money. 



The material against which disinfection is directed is 

 living matter, generally very minute, so much so that even 

 with the highest powers of the microscope it has not yet 

 been seen for some of the most infectious, diseases. 



The difficulty attending disinfection in lay hands is due 

 to the extremely minute size of most pathogenic organisms, 

 such as the untrained mind is quite unable to grasp. It 

 cannot realize that a blade of grass may have on it sufficient 

 virus to infect a whole herd or flock, or that a neglected 

 corner of a pig sty, easily covered with the tip of the finger, 

 may be the means of starting swine fever among a pre- 

 viously healthy batch of pigs. 



Disinfectants are chosen from two groups either chemical 

 or physical, the latter being heat in some form or other. 

 Chemical disinfectants are a large group consisting prin- 

 cipally of acids, salts, and gases. 



Disinfection by acids need not occupy us ; with strong 

 acids like nitric, sulphuric, etc., it is very complete, but the 



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