'202 
DK. W. J. KUSSELL ON THE FOKMATION OF 
now 12 millims. from it. On still fui'ther increasing the distance between the strip 
and the plate, the amount of deposit goes on increasing and travels nearer the centre. 
When raised to 4 millims. above the plate the deposits have met in the centre, and 
when the height between the plates is 7 millims., then the deposit is 15 millims. from 
the edge, and when 10 millims. above the lower plate the deposit is 18 millims. from 
tlie edge, and is central to the large cross (hg. 46). At a distance of 15 millims. the 
strip no longer produces any effect, the ordinary large cross forms. 
In order to ascertain whether any figure coidd be formed by a dusty atmospliere 
^\■hen in motion, magnesium was burnt in an asbestos tube, while a current of air was 
being drawn through it. The asbestos tube Avas attached to a glass tube, 32 millims. 
in diameter, and in this tube pieces of glass 
of different lengths were introduced for 
tlie figures to form on. It was found that 
a peculiar and characteristic figure was 
always produced. It consists of a multi¬ 
tude of dust streams which unite into a 
single stream, as shown in fig. 47. If the 
tube be wider, the same picture is formed liy increasing the amount of air drawn through 
the tube. It may also be stated that if the dust atmosphere be violently disturbed by 
means of a stirrer, while the dust is settling on the plate, it produces no alteration of 
tlie figure wdiich is forming witliout the stirrer comes very close to the plate. 
Tins figure, formed in the tube, is probal)ly of a somewhat different character froui 
the previous ones, for it forms quite as readily when the plate is not warmed as it 
does when it is warmed. 
When the dust is obtained by ljurning 
magnesium, the magnesia formed undergoes 
some curious changes. The figure when first 
formed lies loosely on the plate, the slightest 
friction will remove it. If, however, it lie left 
■exposed to the air, it loses its silvery whiteness 
nnd becomes more and more attached to the 
glass, so that after about a week or fortnight 
tlie figure may be lightly rubbed without its 
being removed. Again, the magnesia itself 
undergoes a change of foiin immediately after 
its production. If the dust be collected at 
■once, that is, while the magnesium is still 
burning, and be examined under a microscope, 
it will be seen that it is made up of small separate irregular-shaped particles about 
<n005 millim. long (fig. 48) ; but if the dust be collected after the combustion is 
■over, and it has stood for one or two minutes, then its form is different, for ft now 
Fia;. 48. 
