718 THE MOLECULAR STKUCTURE OF MATTER. 



aud it seems certaiu that bayonets, swords, and gnus are liable to 

 changes which may account for some of the unsatisfactory results which 

 have manifested themseh^es at tests repeated after a considerable inter- 

 val of time. As all these things have been hardened and tempered, 

 there must necessarily have been considerable constraint put upon the 

 freedom of motion of the particles. This constraint has gradually been 

 overcome, but at the expense of the particular quality of the steel which 

 it was originally intended to secure. 



I have now laid before you the views respecting the constitution of 

 matter which I think are gaining ground, which explain many phe- 

 nomena with which we are familiar, and which will serve as guides in 

 our treatment of metals, aud especially of alloys; but I must admit that 

 the subject is still by no means clear, that a great deal more definition 

 is wanted, and that we are still awuiting the advent of the man who 

 shall do for molecular [)hysics what Newton did for astronomy in ex- 

 plaining the structure of the universe. 



PETROLEUM. 



One of the most remarkable features of the last thirty years is the 

 introduction of petroleum, and the wonderful development to which the 

 trade in it has attained. Under the generic name of petroleum is 

 embraced a variety of combinations of carbon and hydrogen, each of 

 which is distinguished by some special property. At ordinary tempera- 

 tures and pressures some are gaseous, some are liquid, and some solid, 

 and most are capable of being modified by suitable treatment under 

 various temperatures aud pressures. The employment of petroleum in 

 the arts is still extending rapidly. Used originally for illuminating- 

 purposes, it is now employed as fuel for heating furnaces and steam- 

 boilers, and as a working agent in heat engines : valuable medicinal prop- 

 erties have been discovered ; and as a lubricant it stands unrivalled. 



As a working agent in heat engines it is emidoyed in two ways : First, 

 as a vapor generated from the liquid petroleum contained in a boiler, 

 very much in the same way as the vapor of water is used in an engine 

 with surface condenser, the fuel for producing the vapor being also 

 petroleum. Very signal success has been obtained by Mr. Yarrow and 

 others in this mode of using mineral oil, especially for marine purposes 

 and for engines of small power; there seems to be no doubt that by 

 using a highly volatile spirit in the boiler a given amount of fuel will 

 produce double the power obtainable by other means, and at the same 

 time the machinery will be lighter aiul will occupy less space than if 

 steam were the agent used. The other method is to inject a very line 

 spray of hot oil associated with the proper quantity of air into the cylin- 

 der of an ordinary gas engine, and ignite it there by means of an elec- 

 tric spark or other suitable means. Attempts to use oil iji this way 

 date back many years, but it was not till 1888, that Messrs. Priestman 

 Brothers exhibited at the Nottingham show of the Royal Agricultu- 

 ral Society an engine which worked successfully with oil, the flashing 

 point of which was higher than 75^ F., and was therefore within the 



