The portable dust-counting apparatus described in a previous communication to this Society was designed with a view of making observations on the air at situations where it would be inconvenient to work with the larger laboratory apparatus; and also to enable these observations to be made under conditions more favourable for avoiding local impurities than is possible when working in a house. As the construction of this portable apparatus was completed just as I was about to start for the

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... nt, the opportunity seemed a favourable one for continuing the investigation into the number of dust particles in the atmosphere, and extending our knowledge on the subject in other countries. The portable apparatus is reduced to such dimensions that it adds but little to one's personal luggage, and can be easily carried to' the place where the observations are to be made.