126 ATOMS AND SUNBEAMS. 



actually perceive are vibrations comuiniiicatcd tlirouj;li the air to the 

 auditory apparatus. We can trace these aerial vibrations back to their 

 source, and we find they originate from the quivering of the violin 

 under the influence of the bow of the performer. Were it not for these 

 vibrations of the instrument the aerial vibrations would not be pro- 

 duced, and the corresponding souiuls would not be heard. Far more 

 delicate than the atmospheric; waves of sound are the .Ttherial waves 

 corresponding to light or to heat, but none the less nmst these latter 

 also originate from the imx)ulse of some vibrating mass» It is thus 

 apparent that a hot piece of iron, however still it may seem, must be 

 animated by an excessively rapid molecular movement. Xor is the 

 validity of this conclusion impaired even if the iron be at ordinary 

 temperature. We know that a body which is no hotter than the sur- 

 rounding bodies is still incessantly radiating heat to them and receiving 

 heat from tliem in return. Thus we are led to the conviction that a 

 piece of iron, whatever be its temperature, must consist of atoms in a 

 state of lively movement. The important conclusion thus drawn with 

 regard to iron may be equally stated with resi)ect to every otlier solid, 

 or, indeed, every other body, whether solid, liquid, or gaseous. All 

 matter of every description is not only known to be composed of mole- 

 cules, but it is also now certain that those molecules are incessantly 

 j)erforming movements of a very complex type. 



A closer study of this subject will be necessary for our present pur- 

 pose, and it will be convenient to examine matter in that state in which 

 it IS exhibited in its very simplest type from the molecular point of 

 view. This condition is not presented, as might at first be supposed, 

 when the matter is solid, like a diamond, or like a piece of iron. Even 

 in a liquid the complexity of molecular constitution, though somewhat 

 less than in the case of a solid, is still notably greater than in matter 

 which has the gaseous form. The air that we breathe is matter almost 

 of the most simple kind, so far as molecular constitution is concerned. 

 It should however be noted, that as air consists of a mixture, it would 

 be better for our purpose to think of a gas isolated from any other ele- 

 ment. Let us take the case of oxygen, the most important constituent 

 of our atmosphere. 



Like every other element, oxygen is composed of molecules, and those 

 molecules are in a state of rapid motion. It might be expected that 

 the afitinity by which the different molecules were alli(Hl in the case of 

 a gas should be of the simplest nature, and this is indeed found to be 

 the case. Notwithstanding that oxygen is an invisil)le body, and m^t- 

 withstanding that the molecules are so excessively minute as to be 

 severally quite inappreciable to our senses, yet we have been able to 

 learn a great deal with regard to the constitution of the molecules of 

 this gas. The mental eye of the philosopher shows him that though 

 the oxygen with which a jar is filled appears to be perfectly quiescent, 

 yet that quiescence has there no real existence. He knows that oxygen 



