โ Scribed by C.B. Bazzoni
No coin nor oath required. For personal study only.
THE dust figures which occur on the walls and ceilings of rooms frequently outlining the laths and other underlying structures have been commented upon at different times in scientific periodica1s.l The figures have been attributed by some to deposits of moisture, and by others to electrostatic or thermal effects.
These figures are of two types. Those of the first type are, in the opinion of the writer, to be referred to local temperature differences on the wall surfaces which cause corresponding irregularities in the moisture deposition from the currents of damp air flowing over them. Areas with a temperature below the dewpoint of the vapor in the air become covered with a water film which takes up the dust particles from the air by ordinary adhesion.
This dust deposit is possibly increased through deposition resulting from the checkin g in the velocity of the air current due to, the greater surface friction over the wet areas. This simple theory suffices to explain all of the dust figures outlining underlying structures ordinarily observed.
The most common dust figure of this type is that showing up the laths and beams under the plaster.
The black areas making LIP these figures are found to be between the laths. These figures are never marked, if at all present, in interior rooms or on the ceilings of lower story rooms where upper stories are heated. They occur fully developed on the ceilings of rooms covered by a cold attic and on the inner sides of exterior walls. The more rapid convection and radiation of heat from the areas not backed by lath or beam produce the necessary local temperature differences for the production of the irregular water films. A striking illustration of this theory is shown where beams contain tie-bolts or large lag screws the heads of which are covered by the plaster. The locations of these iron bolts are indicated in the dust figures
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