Certainly, these instances show a good balance of lives saved, not only under the adverse conditions of the war, but also in Egypt, India, and the United Kingdom. But the bacteriological work on typhoid fever has been directed also to the working out of a very different problem: and that is the method of diagnosis which is called "Widal's reaction." The practical uses of this reaction are of the utmost importance. It is the outcome of work in different parts of the world--by Wright and Semple and Durham in England, Chantemesse and Widal in France, Pfeiffer and Kolle and Grüber in Germany, and many more. The first systematic study of it was made by Durham and Pfeiffer; and Widal's name is especially associated with the application of their work to the uses of practice. Admirable accounts of the whole subject are given by Dr. Cabot in his book, _The Serum-Diagnosis of Disease_ (Longmans, 1899), and by Mr. Foulerton in the _Middlesex Hospital Journal_, October 1899 and July 1901. Widal's reaction is surely one of the fairy tales of science. The bacteriologist works not with anything so gross as a drop of blood, but with a drop of blood fifty or more times diluted; one drop of this dilution is enough for his purpose. Take, for instance, an obscure case suspected to be typhoid fever: a drop of blood taken from the finger is diluted fifty or more times, that the perfect delicacy of the test may be ensured; a drop of this dilution is mixed with a drop of nutrient fluid containing living typhoid bacilli, and a drop of this mixture of blood and bacilli is watched under the microscope:-- "The motility of the bacilli is instantaneously or very quickly arrested, and in a few minutes the bacilli begin to aggregate together into clumps, and by the end of the half-hour there will be very few isolated bacilli visible. In less marked cases, the motility of the bacilli does not cease for some minutes; while in the least marked ones the motility of the bacilli may never be completely arrested, but they are always more or less sluggish, while clumping ought to be quite distinct by the end of the half-hour." The result of this clumping is also plainly visible to the naked eye, by the subsidence of the agglutinated bacteria to the bottom of the containing vessel: and thus an easy practical mode of diagnosis is afforded by it.