of Edinhurgli, Session 1883-84. 
445 
is not perfect. Still it is thought that this repelling action at these 
surfaces must have some influence, and it seems possible it may 
explain some climatic effects, as it is evident that the lungs will he 
much better protected in such places as Davos Platz, where the air 
is cold and dry, and the repelling forces at a maximum, than at 
places like Madeira, where the air is warm and moist, and these 
forces are at a minimum. This point can, however, only be 
determined satisfactorily by anatomical examinations of lungs 
which have lived under the different conditions. 
In the experiments it was observed that dust not only tended to 
move away from hot surfaces, but also that it was attracted by cold 
ones, and attached itself to them. To study this effect, glass plates 
were put in different positions near the hot platinum wire. Very 
beautiful impressions of the dark plane can be obtained by placing 
a piece of glass vertically and transversely over the hot wire. The 
hot air in flowing over the glass deposits its dust on the surface of 
the plate, leaving a clear line in the middle, indicating where the 
dustless air of the dark plane had passed. In this way the dust 
is trapped on the glass to which it adheres with some firmness, and 
not only the impressions, but the dark planes themselves, may thus 
be preserved.* 
Other experiments, to study the repulsion and attraction of hot 
and cold surfaces, were made by placing glass plates on both sides 
of the hot wire. An interesting result was obtained when the plates 
were about 1 mm. apart. Using magnesia powder, the particles 
could be seen rising in the current and approaching the hot wire ; 
they were then observed to be violently repelled towards the cold 
surfaces, to which they adhered. If there was sufficient difference 
of temperature, not a single particle of dust was carried by the 
current past the hot wire. 
A thermic filter is then described. In this filter the air is passed 
through the space formed between two concentric tubes. One tube 
is kept cold by a stream of water, and the other heated by means of 
steam or a flame. This instrument was shown in action. One end 
* Specimens of these trapped dark planes were shown at the meeting. 
Some of them made of white powder deposited on blackened glass, others of 
charcoal deposited on opal glass. 
