booD Preservatives. 91 



nary articles of food for one of the common preservatives, when 

 it is present in the amount generally used or absent altogether. 

 But sometimes the preservative is present in small amount or is 

 used in combination with other substances ; there may be inter- 

 fering substances present; and difficulties may arise during the 

 manipulation, all of these complicating the work and tending to 

 throw doubt upon the results. 



