This chapter shows that inhaled dust can penetrate to the air cells of the lungs. It provides the evidence for this statement, and the discussion of the relative share of this and other methods of infection. The chapter discusses the operation of infection by dust and by spray, as far as possible in the historical order of the most important experiments that have been made. Dried sputum is much more likely to cause infection, as, owing to the negligence with which the expectoration of phthisical patients is treated, it must evidently enter the atmosphere in considerable quantity. Heymann made experiments to determine the local dispersion and limitation of the sputum drops. Experiments were also made by Heymann on the duration of suspension in air of droplets containing tubercle bacilli. Tubercle bacilli have been frequently found in the dust of railway carriages, omni-buses, and tram-cars.