

By WILLIAM PHILLIPS MENDHAM, Associate of the 

 Society of Telegkaph-Engineees and Electricians. 



(Abstract.) 



OWING to tlio extreme mobility of the particles of dust 

 with wliioli our atmosphere is laden, it is a very easy 

 matter to displace them, as they are susceptible to very 

 gentle currents of air, but if left to themselves will, under 

 certain conditions, quietly settle till again disturbed ; they 

 may also be destroyed by burning. 



A beam from a lantern reveals the presence of these 

 particles, and if a hot flame or a heated body be hold be- 

 neath the beam, a black space is produced across the beam, 

 owing to their combustion by the flame and displacement by 

 the ascending current of hot air from the heated body. 



The most probable explanation of this behaviour of the 

 dust-laden air seems to be that the air immediately above 

 the source of heat becomes rarefied, and hence rises more 

 quickly than that air without its influence. The dust parti- 

 cles are not able to rise so quickly as the rarefied air which 

 contained them, and are consequently loft behind. 



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