_Answer._ Probably in some locomotives the pressure of the driving-wheels on the rails is sufficient to partly crush the latter. QUESTION 369. _What effect has the nature of the materials in contact on the friction?_ _Answer._ The amount of friction and also the lubrication is very much influenced by the nature of the bearing surface and also by the material used as a lubricant. Some metals, such as brass and other alloys, are much less liable to abrasion and seem to retain lubricants on their surfaces better than other metals, and are therefore much used for journal and other bearings. Some substances, especially oils, are good lubricants, while other materials of apparently similar nature are not. The reason why these materials possess these properties while others are without them is not known, and the value of any material as a lubricant, or the degree to which another will resist friction without abrasion, can only be tested by experiment. PART XIX. COMBUSTION. QUESTION 370. _What is meant by combustion?_ _Answer_. By combustion is meant the phenomenon ordinarily called burning, as when a piece of wood or coal or a candle is burnt. In reality combustion is a union of one of the “_chemical elements_,” oxygen, of which the atmosphere is composed, with the elements which constitute the fuel. QUESTION 371. _What is meant by the term “chemical element?”_ _Answer_. The science of chemistry has demonstrated that nearly all substances by which we are surrounded are composed of certain other substances, which latter, as far as is now known, are not compounds, and are therefore called _elementary substances, or chemical elements_. Thus the air by which we are surrounded is composed of two gases, called nitrogen and oxygen; water is composed of hydrogen and oxygen, and coal chiefly of carbon and hydrogen. There are now over sixty of these elementary substances known. From no one of them have chemists been able to extract any material excepting the substance itself. These elementary substances will combine with others so as to form what is apparently a new material, but on weighing it it will be found that the weight of the new material is greater than the original elementary substance, showing that something was added to it which effected the change.[87] [87] “The New Chemistry,” by J. P. Cooke, Jr. QUESTION 372. _To what fact is this combination or combustion of elementary substances due?_ _Answer_. It is owing to the fact--the exact reason for which is perhaps not yet understood fully--that the atoms of the elementary substances of which fuel is composed, that is hydrogen and carbon, and the atoms of oxygen, which forms part of the atmosphere by which we are surrounded, attract each other with great energy when they are excited into activity by the application of heat. QUESTION 373. _What phenomenon always attends chemical combination of substances?_