PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 477 



ajrreeinir with those of Willi and Cruo-er in Trinidad, where fossil 

 plants are sometimes found partly converted into petroleum and 

 partly into lignite. 



Dr. Hunt regards the process by which animal and vegetable 

 hydrocarbonaceous tissues have been converted into solid or 

 liquid bitument, as a decay or fermendation under conditions in 

 which atmospheric oxygenation is excluded, so that the maximum 

 amount of hydrogen is restrained by the carbon ; and as repre- 

 senting one extreme of a process, the other of which is found in 

 anthracite and mineral charcoal, the two conditions beins: antajr- 

 onistic and excluding each other, and the production of petroleum 

 implying, wdien complete, the disappearance of the organic tissue. 

 Hence pyroschists, the so-called bituminous shales, and coal are 

 not found together with petroleum, but in separate formations, 

 and it is to be borne in mind that the epithet bituminous, applied 

 to the former bodies is a mistaken one, since they seldom or never 

 contain any bitumen, although like all fixed organic bodies, they 

 yield hydrocarbons by destructive distillation. The fallacy of 

 the notion which ascribes petroleum to the action of subterranean 

 heat on strata holding coal and pyrochists, was exposed, and it 

 w^as remarked among arguments founded upon the impermability 

 of man}^ of the petroleum-bearing strata, that the oil of the 

 Trenton limestone occurs below the horizon of any pyroschists or 

 other hydrocarboneous rocks. 



Among improvements proposed and new inventions described, 

 may be mentioned, first, the paper by President Barnard describing 



A New Method of Illu3iinating Opaque objects under powers 

 OF THE Microscope. 



The author after explaining that when powers are used the dis- 

 tance to the object to be magnified was so small it Avas found 

 almost impossible to illuminate. The means now used for illu- 

 minating is the Lueberkuehr mirror — a highly polished conical 

 plate. The author made a series of explanations with this mirror, 

 placing it inside the tube above the lowest object glass and arrang- 

 ing it so that the light will pass to the mirror and then dowai upon 

 the object. In order to fully carry out his idea it was necessary 

 to grind the lower or plano-convex lens in a new way. 



Prof. Perkins, of Union College, after speaking favorably of the 

 improvements described, proposed to perforate the shoulder of 

 the main tube at the place where the object glass tube is screwed 



