188 
DR. W. J. RUSSELL ON THE FORMATION OF 
surrounding atmosphere. When filling the receiver with the magnesia smoke, it 
should be turned round over the burning magnesium, so as to render the whole of the 
inside of as uniform a temperature as possible, and before placing the receiver over 
the plate, after the combustion is over, it should be allowed to stand for about a 
minute, so that aiiv coarse particles may settle. First, with regard to cases in which 
the plate is lower in tempeivature than the surrounding atmosphere. If the jdate he at 
a temperatuie of 19 C., and the maximum temperature of the dust atmos])here be 
24°, then a iiearly uniform deposit is produced, but at tlie corners of the plate there 
is a short line of deposit, and along the sides there is somewhat less deposit (fig. 5). 
If the jilate be Avarmer, 20‘G°, and the atmos])hei'e 24°, then the above characters are 
still further developed and a bag-shaped deposit is formed (hg. 6), and this is 
characteristic of AA'hat takes place Avhen the plate is beloAA’ the temperatuie of the 
atmosphere, but sufficiently Avarm to act. When the plate is slightly Avarmer than 
the dust atmosphere, 1-8° for instance, then again a figure is produced similar in 
character to the last one, but a further deA'elopment of it (fig. 7). If the plate he 
