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42
so exposed for not less than half-an-hour after the sweeping had
ceased.
The tacky surface of the mastic varnish caused all dust falling
on the face of the plate to adhere, and after drying to become fixed.
Immediately after exposure the plates were placed in dust-proof
boxes until thoroughly dry, and thus a permanent record was
secured of the dust actually falling on the plates during the period
of exposure. For the purposes intended, it was considered that
sufficient accuracy would be attained by classifying the degree of
dustiness of the plates according to the following rough scale:—
Degree.
0
1 = Practically no dust.
2 = Evident dustiness.
3 - Very dusty.
4 = Excessive dustiness.
The tests were carried out as stated above, on hard and on soft
wood floors.
A control was kept in an adjoining room of the same school as
nearly as possible identical with the room in which the dust allayer
was used, the plate being exposed for the same period during and
after sweeping.
The results of the observations are represented graphically in
the accompanying diagrams.
The stars connected by dotted lines represent the degree of dustiness
recorded on the plate while sweeping the untreated floors in the
ordinary way and the figures at the top of the diagram represent the
number of days which had elapsed since the day on which the floors
were last washed and scrubbed.
The dots, triangles or small circles connected by the continuous line
represent the degree of dustiness recorded on the plate while sweeping