THE RADIOMETER. 7 



grounds for believing that they are sufficiently accurate to give us a 

 true idea of the order of the quantities with which we are dealing ; and 

 it will be seen that' although the numbers required to express the rela- 

 tions to our ordinary standards are so large, these molecular magnitudes 

 are no more removed from us on the one side than are those of astron- 

 omy on the other. 



Passing next to the velocity of the molecular motion, we find in that 

 a quantity which, although large, is commensurate with the velocity of 

 sound, the velocity of a rifle-ball, and the velocities of many other mo- 

 tions with which we are familiar. We are, therefore, not comparing, 

 as before, quantities of an utterly different order, and we have confi- 

 dence that we have been able to determine the value within very nar- 

 row limits of error. But how surprising the result is ! Those molecules 

 of hydrogen are constantly moving to and fro with this great velocity, 

 and not only are the molecules of all aeriform substances moving at 

 similar, although differing, rates, but the same is equally true of the 

 molecules of every substance, whatever may be its state of aggregation. 



The gas is the simplest molecular condition of matter, because in 

 this state the molecules are so far separated from each other that their 

 motions are not influenced by mutual attractions. Hence, in accord- 

 ance with the well-known laws of motion, gas-molecules must always 

 move in straight lines and with a constant velocity until they collide 

 with each other or strike against the walls of the containing vessel, 

 when, in consequence of their elasticity, they at once rebound and 

 start on a new path with a new velocity. In these collisions, however, 

 there is no loss of motion, for, as the molecules have the same weight, 

 and are perfectly elastic, they simply change velocities, and whatever 

 one may lose the other must gain. 



But if the velocity changes in this way, you may ask, What meaning 

 has the definite value given in our table ? The answer is, that this is 

 the mean value of the velocity of all the molecules in a mass of hydro- 

 gen gas under the assumed conditions ; and, by the principle just stated, 

 the mean value cannot be changed by the collisions of the molecules 

 among themselves, however great may be the change in the motion of 

 the individuals. 



In both liquids and solids the molecular motions are undoubtedly as 

 active as in a gas, but they must be greatly influenced by the mutual 

 attractions which hold the particles together, and hence the conditions 

 are far more complicated, and present a problem which we have been 

 able to solve only very imperfectly, and with which, fortunately, we 

 have not at present to deal. 



Limiting, then, our study to the molecular condition of a gas, pict- 

 ure to yourselves what must be the condition of our atmosphere, with 

 its molecules flying about in all directions. Conceive what a molec- 

 ular storm must be, raging about us, and how it must beat against 

 our bodies and against every exposed surface. The molecules of our 



