DEFINITE FIGURES BY THE DEPOSITION OF DUST. 
555 
same deformations as shown in figs. II, 12, and 13 can he equally well produced by 
heating the receiver by means of the hand held on it. 
Turning now to the results obtained by Dr. Russell when the plate was rested on 
a hot cylinder of metal, as shown in fig. 14. Here everything is reversed; black 
centre and black diagonal arms where in the other figures it was white, and white to 
the edges of the plate where before it was black. How, it may be asked, stands the 
explanation now; where is the protecting effect of the dustless film from under the 
plate ? For explanation let us turn to experime 2 it. When the beam of light is 
turned on to this new condition of matters we find the air circulation is all changed. 
The dustless film from under the plate no longer turns round the edge and flows 
horizontally over the upper surface, but the large amount of hot air coming from 
the hot cylinder below the plate causes the dustless film to rise straight up from tlie 
edge, and an induced current of air is seen flowing over the plate from the centre to 
the edge, depositing its dust as it goes. It is only at tlie corners of the plate, where 
the mutual influences of the neighbouring currents and the amount of hot air is less, 
and where the currents approach and bring the dustless film over the plate, that 
there is any protection. 
Both of Dr. Russell’s figures, shown in figs. 9 and 14, were produced by somewhat 
similar conditions. In both cases a hot body was placed beneath the plate, but in 
the case shown in fig. 9 the hot cylinder was placed some distance below the plate 
and only heated to 55° (I; whereas, in the other case, the hot cylinder was at a 
temperature of 150° C., and the plate rested on it. Referring to fig. 9, Dr. Russell 
points out that as the temperature of the body underneath the plate is increased the 
amount of deposit also increases, and idtimately the figure of the cross disappears ; })ut, 
as will be seen from fig. 14, it reappears in a reversed form when the temperature is 
high enough and the plate rests on the hot body, all of which is easily understood by 
what has been said above. 
Figs. 15 and 16 do not call for any sjjecial observation. Fig. 17 is interesting as 
showing the effect when the plate is cold and the currents are produced by an 
influence above the plate. In this case the currents flow over the cold plate 
towards the hot cylinder placed at the centre. As these currents do not come from 
the under side of the plate, they do not have a dustless film. So tlje plate has dust 
deposited all over it, but the figure of the cross can still be seen and is produced l)y 
the currents from the different sides flowing towards the centre, meeting over tlie 
diagonals, and causing the calm depositing areas as in the previous cases, only more 
feebly. In fig. 18 the white deposit round the cold cylinder is caused by the cold 
air flowing down the cylinder and forming a calm dust-depositing area round it. 
The effect of placing the plate in a sloping position is shown in figs. 19, 20 and 21. 
These alterations in the forms of the deposited dust are evidently due to the slope of 
the plate interfering with the flow of the dustless film from underneatli the plate 
and to the change produced by the slojDe on the currents over the upper surface. 
4 B 2 
