130 HEAT. 



no doubt largely contributed to its universal acceptance. The hypothesis 

 in its original form may be stated thus : A given mass of an element 

 contains a definite number of exactly similar indestructible particles, 

 termed atoms, these particles being exceedingly minute and exceedingly 

 numerous in any volume which comes within the range of our senses. 

 Each atom of a given element has a definite mass, which is always the 

 same for the same element. 



Though this form of the hypothesis is sufficient for many purposes, 

 the relations which exist between the atomic weights and the other 

 properties of the various elements lead to the supposition that the atoms 

 of different elements are not simple indestructible particles unrelated to 

 each other, but are built up of some common material, the differences 

 being in the quantity and arrangement of this common material. Were 

 an atom of iron, for instance, entirely unrelated to an atom of platinum, 

 had they existed as they are through all time, it would be difficult to 

 account for such a relation as the equality of their atomic heats. It is 

 now generally believed that such relations may ultimately be deduced 

 from the supposition that the atoms are complex bodies, containing still 

 smaller particles or corpuscles, all alike in all elements, but different in 

 number and arrangement in the different atoms. Recent experiments, 

 especially certain experiments on the discharge of electricity in rarefied 

 gases, have given great weight to this supposition, since they may be to 

 some extent explained if the atoms are regarded as being split up in the 

 discharge, giving out corpuscles, the same for different gases. In this 

 chapter, however, we shall not go beyond the supposition that the atoms 

 are simple bodies. 



In chemical compounds the dissimilar atoms of the constituent 

 elements are supposed to be grouped together to form molecules similar 

 to each other, the masses of the components being in the same propor- 

 tion in the molecules as in the compound as a whole, and the laws of 

 chemical combination find a simple explanation by supposing that the 

 mass of each kind of atom is definite. 



The atoms, even in an elementary substance (in which, so far as we 

 know, they are all similar), are probably to be regarded as grouped, 

 perhaps in pairs, perhaps in greater numbers, and we may fairly extend 

 the term " molecule " to describe a group so formed. 



When we seek to explain the various forms of energy on this atomic 

 or molecular hypothesis, we have further to suppose that the atoms are 

 held together by forces, for work is required to effect chemical decom- 

 position. 



Work, too, is required to alter the volume of any liquid or solid body 

 in its normal condition. Hence, we must regard the molecules also as 

 held together by forces. We may have energy of motion both of atoms 

 and of molecules, and energy of position both of atoms with regard to 

 each other, and of molecules with regard to each other. 



We will now consider the explanation which the atomic hypothesis 

 gives of the phenomena of heat. 



When two solid bodies are pressed together and rubbed, one on the 

 other, work is done against friction and ordinary kinetic energy dis- 

 appears as fast as it is supplied to the bodies. The surfaces in contact 

 are found to rise in temperature. We may suppose that the friction 



