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Let AO A' BOB' be the given conjugate diameters. Draw ■ 
Bi^T parallel to AA'. With the centre B and radius OA 
draw the circle DH/?. In this circle take any radius BD at 
pleasure, perpendicular to which draw the diameter HBA 
Find O' in BD such that 0'H=0'0=0'/i, make 0'^=:0'T= 
00', join 0^ and OT. These lines will be conjugate. To 
find the length of the diameters draw Bm parallel to 0^, and 
Bn parallel to OT, then OM^=OT . Om and 0N^=0^ . On, 
A simple construction with a circle is usually adopted to 
determine OM ON geometrically from these two equations, 
Mr. Arthur Mc.Dougall, B.Sc., invited attention. to a 
specimen of carbon formed upon the roof of a gas retort, 
by the decomposition of the hydrocarbon gas by heat. 
The carbon thus formed resembles graphite in its almost 
metallic lustre, and it was suggested that its mode of 
formation might throw some light upon that of graphite. 
Graphite always occurs in association with rocks which 
have been subjected to igneous action, and may have been 
formed by hydrocarbon gases traversing fissures or dykes 
whilst the sides were in a highly heated state, thus causing 
a deposit similar to that formed in gas retorts. The fact 
that in the latter case an increase of pressure causes a 
greatly increased amount of deposit favours this view, as it 
is extremely probable that any gases existing in the earth's 
crust would be in a state of great tension. 
