96 EECORDS OF THE AUSTRAXIAN 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 

 inlluences 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 sliglitly 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 elTective 

 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. 



