96 
RECORDS OF THE AUSTRALIAN MUSEUM. 
A sunbeam shows how laden with dust is that atmosphere 
which otherwise appears so pure ; this suspended dust is forced 
into a case at every increase of pressure. Before the barometer 
indicates that the pressure has diminished, and that the surplus 
air is once more passing out, the dust has probably settled on our 
specimens and labels ; this interchange of air is going on continu- 
ally and occurs at least twice daily. Apart from barometric 
influences a high thermometer registers a less pressure on the case: 
a fall in temperature increasing the pressure, forces in dust-laden 
air at every point of least resistance. 
Although museum labels have been referred to as showing the 
presence of dust, we need not go beyond our own homes for 
similar indications : — who has not remarked on the streak of dust 
across a glazed picture, consequent on a minute hole in the 
papered back or a slightly puckered mount ; the dust track en- 
gendered in a book by a crumpled leaf or folded plate ; or again 
on a dirty ceiling where the position of the non-porous joists is 
clearly shown by the lighter color of the plaster 1 
It has long been known that when air is passed through cotton- 
wool the dust is filtered out. Starting with this knowledge, which 
has been freely applied, Mr. Teale has materially extended our 
acquaintance with the subject by experimenting with various filter- 
ing mediums and showing how such may be used. It is not my 
province to recount these experiments ; for such my readers are 
referred to the paper before mentioned. The fundamental idea is, 
to allow air to enter freely through a large aperture, guarded by 
a filter suitably mounted. Of those tried, the most effective 
materials were found to be cotton-wool and cotton-demette. 
Our experience at the Leeds Museum was, that very much dust, 
especially that of a coarser nature, might be arrested by employ- 
ing such filters. When first put up they worked admirably, but 
in time the fibres became clogged with dust. If not then taken 
down and brushed, they acted as dust furnishers ; a certain 
amount being forced through the fibres every time the air passed 
into the case. 
Whilst making experiments in the Australian Museum by the 
kind permission of the Curator, who also assisted me in every 
possible way, it occurred to me that the difficulty might be sur- 
mounted in another manner : — by endeavouring to protect the 
case, not from the dust which the pressure forces in, but from 
that pressure itself. 
Adopting the principle of an aneroid barometer, the wall of a 
case or drawer is to be transformed from an inflexible to a flexible 
diaphragm, its contained air separated from the air without by 
an accommodating but impervious membrane ; the ordinary case 
is porous on account of its immobility. 
