ATOMS AND SUNBEAMS. 127 



consists of myriads of molecules identical in weight atid in other 

 features, and darting about one among the other with velocities which 

 vary perhaps between those of express trains and those of rifle bullets. 

 He sees that each little molecule hurries along quite freely for a while 

 until it happens to encounter some other molecule equally bent on its 

 journey, and then a collision takes x>hice. Perhaps it would be more 

 (;orrect to say that what usually happens is that tlie two impinging 

 molecules make a very close approach ; then each of them so A-ehemently 

 attracts the otiier as to make it swerve out of its course and start it off 

 along a path, inclined, it may be, even at a right angle to that which 

 it previously pursued. The molecules in a gas at ordinary pressures 

 are so contiguous that these encounters take place incessantly; in fact, 

 we are able to show that each individual molecule Mill i)robably expe- 

 rience such adventures some millions of times in the course of each 

 second. We are able to calculate the average velocity with which the 

 several molecules move when the gas has a certain temperature. We 

 know how to determine the average length of tlie free j^ath which each 

 molecule traverses in the interval between two consecutive encounters. 

 We are able to trace how all these circumstances would vary if, instead 

 of oxygen gas, we took nitrogen, or hydrogen, or any other body in the 

 same molecular state. It is in fact characteristic of every gas that 

 each molecule wanders freely, subject only to those incessant encoun- 

 ters with other similar wanderers by which its path is so frequently 

 disturbed. If two gases be placed in the same vessel, one being laid 

 over the other, it will presently be found that the two gases begin to 

 blend; ere long one gas will have diffused uniformly through the other, 

 so that the two will have become a perfect mixture just as the oxygen 

 and nitrogen have done in our own atmosphere. The molecular theory 

 of gases exi^lams at once the actual character of the operation by which 

 diffusion is effected. Across the boundary which initially separates 

 the two gases certain molecules are r)rojected from either side, and this 

 process of interchange goes on until the molecules become nniibrmly 

 distributed throughout. 



There is indeed nothing more remarkable than the fact that infor- 

 mation so copious and so recondite can be obtained in a region which 

 lies altogether beyond the direct testimony of the senses. Just as the 

 astronomer staggers our powers of conception by the description of 

 appalling distances and stupendous i^eriods of time, and relies with con- 

 fidence on the evidence which convinces him of the reality of his state- 

 ments, so the physicist avails himself of a likepotent method of research 

 to study distances so minute and times so brief that the imagination 

 utterly fails to realize them. 



In the case of a liquid, the freedom enjoj^ed by the molecules is con- 

 siderably more restricted than in the case of a gas. It would seem that 

 in the denser fluid there can be no intervals of undisturbed travel per- 

 mitted to a molecule ; it is almost incessantly in a state of encounter 



