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of one in twenty, a second drop one in thirty, third 

 drop one in forty, and finally a drop of typhoid 

 einulsion ; this gives a dilution of serum of one 

 in fifty, containing typhoid bacilli. The whole 

 process of dilution takes less than five minutes. 



5. ■ ■ Typhoid Emulsion. — A bacillus should be 

 used that is known to be active. Take a fresh 

 over-night agar culture and make a fairly thick 

 emulsion in broth or salt solution. 



6. Dilution and Time Reaction. — A dilution 

 of one in fifty with a time limit of half-an-hour 

 may be used. With a less diluted serum the time 

 limit must be less. 



7. Whatever time limit and dilution be used, 

 it is very necessary to perform controls from time 

 to time with a variety of other cases to make sure 

 that the agglutination, if produced, is not produced 

 by normal sera. 



The Isotonic Point or Tonicity of the 

 Blood 



If a drop of blood is allowed to drop into a 

 one per cent, solution of salt in a small test tube 

 and stirred up, the uniformly turbid solution will 

 eventually become clear when the corpuscles have 

 settled at the bottom and the supernatant fluid 

 will be unchanged ; if, on the contrary, we add 

 another drop of blood to a little water in a test 

 tube the whole drop is immediately laked, and we 

 have resulting a solution of haemoglobin. The 

 former solution of salt is called hypertonic, the 

 latter solution of water hypotonic. Now, if we 

 start with such a hypertonic solution, one per cent, 

 salt, and proceed gradually to dilute it, we shall 



