In his first experiments Widal used a test-tube in the following manner: The blood to be tested is diluted by one part of it being added to fifteen parts of broth in a test-tube. The mixture is inoculated with a drop of a typical _Bacillus typhosus_ culture. The tube is then incubated at 37° C. for twenty-four hours, after which it is examined. If the reaction be positive, the broth appears comparatively clear, but at the bottom of the test-tube a more or less abundant sediment will be found. This is due to the clumps of bacilli having fallen owing to gravity. If, on the other hand, the reaction is negative, the broth will appear more or less uniformly turbid. For the _apparatus_ required to carry out the simpler methods of bacteriological work reference should be made to the standard laboratory text-books, which furnish all necessary details. A good microscope, with a 1/12 oil immersion lens, is, of course, essential. This can now be obtained for about £16 (Beck, Swift, Baker, Watson, etc.), and the other necessary apparatus is readily obtainable of Baird and Tatlock, Hatton Garden, E. C., and other makers. FOOTNOTES: [1] _The Contemporary Review_, November, 1897, p. 719. [2] Some notable exceptions are found in the work of the Bath and West of England Society, Lord Vernon's model dairy, and the Essex County Council Bacteriological Teaching Laboratory. [3] We propose throughout to use the term _bacterium_ (pl. _bacteria_) in its generic meaning, unless especially stated to the contrary. It will also be synonymous with the terms _microbe_, _germ_, and _micro-organism_. The term _bacillus_ will, of course, be restricted to a rod-shaped bacterium. [4] Migula has recently (1896) suggested that the Schizomycetes should be subdivided into _Coccaceæ_, _Bacteriaceæ_, _Spirillaceæ_ (spirilla, spirochæta), _Chlamydobacteriaceæ_ (Streptothrix, Crenothrix, Cladothrix), and _Beggiatoa_. [5] A one-twelfth oil immersion lens is requisite for the study of the lower bacteria. [6] A _flagellum_ is a hair-like process arising from the poles or sides of the bacillus. It must not be confused with a _filament_, which is a thread-like growth of the bacillus itself. [7] A "pure culture" is a growth in an artificial medium outside the body of one species of micro-organism only. [8] Some pathogenic germs (suppuration and typhoid) can withstand freezing for weeks. [9] G. J. Romanes, _Darwin and After Darwin_, vol. ii., 231.