128 ATOMS AND SUNBEAMS. 



with some other simihir object. When a molecule iu a liquid breaks 

 away from its nssociatiou with one group, it is only because it has 

 entered into alliance with another. As however two liquids will very 

 frequently blend if so placed that diffusion be possible, we have a proof 

 that, though the transference of a particular molecule through the 

 liquid may be comparatively slow, yet it will gradually exchange asso- 

 ciation with one group for association with another, and may in this 

 way travel throughout any distance to which the liquid extends. 



In the case of a solid there is still further limitation imposed on the 

 mobility of each separate molecule. It is now no longer permitted to 

 make excursions throughout the entire volume of the body. Each 

 molecule is in rapid motion, it is true, but those movements are confined 

 to gyrations within miiuitely circumscribed limits. Two solids placed 

 iu contact do not generally diffuse one into the other, the incapacity for 

 diffiision being the direct (jonseqaence of the inferior degree of mobility 

 possessed by the molecules in this condition of matter. 



It is known that the immediate effect of the application of heat is to 

 increase the velocities with which the molecules move. Apply heat, 

 for instance, to tlie water in a kettle; the moving molecules of water 

 are tliereby stinuilated to even greater activity and it will occasionally 

 happen that the velocity thus acquired by a molecule becomes so great 

 that the little particle will swing clear away from the influence of the 

 other molecules with which it had been associated. When this takes 

 place in the case of a sufficient number of molecules, they dart freely 

 from the surface of the liquid, thus producing the effect which in our 

 ordinary language we describe as giving off steam. If therefore a 

 volume of gas be heated, the velocities with which its molecules are 

 animated will be in general increased. As the molecular velocities 

 throughout the extent of the gas are on the whole augmented, it is 

 quite ]»laiii tliat the intensities of the shocks experienced by the mole- 

 cules in their several encounters will be also accentuated. The more 

 rapidly moving particles will strike each against the other with increased 

 violence, and the contemi)latioii of this single fact leads us close to one 

 of nature's greatest secrets. 



Let us think of the abounding heat wliich is dispensed to us from 

 the sun. That heat comes, as we know, in the form of undulations 

 imparted to the ;ether by the heated matter in the sun, and transmit- 

 ted thence across space for the benefit of the earth and its inhabitants. 

 I have already explained that these vibrations in the a'ther must take 

 their rise from molecular movements, and it is important to notice 

 that the character of the vibrations in the a'ther enables us to learn to 

 some extent the precise description of molecular movements which 

 alone would be competent to produce the particular vibrations corre- 

 sponding to radiant heat. At first it might be thought that it Avas the 

 rapid movements of translation of the molecules themselves, as entire 

 if extremely minute bodies, whicli caused the a^.therial vibration, but 



