278 MR. JOHN LEIGH ON THE FORMATION OF COAL. 



their decomposition, with reference to their amount and their 

 quality; an examination of the nature and bulk of the pro- 

 ducts eliminated by distillation at different temperatures ; 

 an examination of the results of distilling coal in aggregated 

 masses and in thin leyers; an examination of the relative 

 effects of heating coal in a thin layer on a sufficiently hot 

 surface, with a comparatively cool arch above, as in a D 

 shaped retort, with the bottom alone heated; and the effects 

 of surrounding the material with a heated surface, as in the 

 round retorts heated all round. 



