THE MOLECULAR STRUCTURE OF MATTER. 719 



category of safe oils. The engine exhibited was very like an ordinary 

 Otto gas-engine, and worked in exactly the same cycle. A pninp at 

 the side of the engine forced air into a stiiall receiver at a few pounds 

 pressure to the square inch. Tlie compressed air, acting b}^ means of 

 a small injector, carried with it the oil in the form of fine spray, which 

 issued into a jacketed chamber heated by the exhaust, in which the oil 

 was vaporized. The mingled air and oil was thus raised to a temper- 

 ature of about 300^, and was then drawn, with more air, into the cylin- 

 der, where, after being compressed by the return stroke of the piston, 

 it was exploded by an electric spark, and at the end of the cycle the 

 products of combustion were discliarge<l into the air after encircling 

 the spray chamber and parting witli most of their heat to the injected 

 oil. Tlie results of careful expi'riments made by Sir William Thomson 

 and myself on different oc(;asions were, that 1.73 pounds of petroleum 

 were consumed per brake horse-power per hour; but the combustion 

 was by no means perfect, for a sheet of paper held over the exhaust pipe 

 w\is soon thickly spattered with spots of oil. 



The enormous consumption of petroleum and of natural gases fre- 

 quently raises the question as to the i)robability of the proximate 

 exhaustion of the supply : and without doubt many fear to adopt the 

 use of oil, from a feeling that if such use once beconu's general the de- 

 mand will exceed the production, the price will rise indefinitely, and 

 old methods of illumination, and old forms of fuel, will have to be re- 

 verted to. From this i)oint of view it is most interesting to inquire 

 what are the probabilities of a continuous supply ; and such an investi- 

 gation leads at once to the question, " What is the origin of petroleum ? " 



In the year 1877, Professor Mendeleef undertook to answer this ques- 

 tion; and as his theory appears to be very little known, I trust you will 

 forgive me for laying a matter so interesting before you. Dr. Mendel6ef 

 commences his essay by the statement that some persons assume (with- 

 out any si)ecial reason excepting perhaps its chemical composition), that 

 nai)htha, like coal, has a vegetable origin. He combats this hypothesis, 

 and points out, in the first place, that naphtha must have been formed 

 in the depths of the earth. It could not have been produced on the sur- 

 face, because it would have evai>orate<l ; nor over a sea bottom, because 

 it would have floated up and been dissipated by the same means. In 

 the next i)lace he shows that na])htha must have been forme<l beneath 

 the very site on which it is found, — that it can not have come from a 

 distance, like so many other geological deposits, and for the reasons 

 given above, namely, that it could not be water-borne, and could not 

 have flowed along the suiface, while in the sui)erficial sands in which 

 it is generally found no one has ever discovered the presence of organ- 

 ized matter in sulliciently large masses to have served as a source for 

 the enormous quantity of oil and gas yielded in some districts; and 

 hence it is most i)robable that it has risen from much greater depths 

 under the influence of its own gaseous pressure, or fioated uj) upon the 

 surface of water, with which it is so freciuently associated. - - 



