A great many different methods are in use for determining the total amount of solid matter present in a given volume of air, counting the number of particles, or gathering samples for microscopic examination. Thus a known volume of air may be drawn through a filter of cotton wool or bubbled through distilled water, and the dust detained by the cotton or deposited in the water may be weighed. In certain types of apparatus the air is drawn or forced against a plate or tube coated with glycerin, oil, varnish, gelatin, or other adhesive surface, to which the dust remains attached. Several devices depend for their operation upon the fact that when a volume of confined air is cooled by expansion a point is eventually reached at which the water vapor present condenses to form a fog, each droplet of which is supposed to have a single particle of dust as its “nucleus.” This is the principle involved in the well-known Aitken dust counter, which has been so extensively used in different parts of the world, and has furnished most of the impressive statistics of air dustiness found in textbooks and reference books. Thus, from indications supplied by this instrument, it is stated that a cubic inch of town air contains 50,000,000 particles of dust; that a room, near the ceiling, was found to contain 88,000,000 particles per cubic inch; and that a cigarette smoker sends 4,000,000,000 particles into the air at every puff. Recent authorities are inclined to look upon these figures as misleading, for the reason that the nuclei counted with Aitken’s instrument are probably so infinitesimal in size that they hardly deserve to be called dust; indeed there is good reason to believe that an indefinitely large proportion of them may actually be molecules of gases.