176 J. MORISON : ADDEESS 



particles of another kind of matter. Thus gases may pass 

 through unglazed pottery or heated iron. 



When we try to look into the real nature of matter, one of the 

 first things to strike us is that it must either be infinitely 

 divisible or that there must be a limit to its divisibility. 

 Anaxagoras and some of the old G-reek philosophers took the 

 former view, that matter could be divided ad infinitum, and that 

 after any number of divisions, no final particle could ever be 

 arrived at which had not two halves, and was not capable of 

 subdivision into smaller pieces. I may mention that this theory 

 is quite incapable of explaining the observed phenomena. 

 Democritus with other old Greek thinkers, on the other hand, 

 held that matter was not infinitely divisible, but that it was 

 composed of ultimate particles which he called atoms, and which 

 were eternal, uncreatable, and indestructible. This atomic 

 hypothesis was afterwards adopted by the famous Roman 

 pliilosopher-poet Lucretius, and treated at length by him in 

 his great work ' De Rerum Natura.' It was revived in modern 

 times, and finally assumed a definite shape as the " Atomic 

 Theory " of John Dalton, the celebrated chemist and physicist, 

 and was promulgated by him at the beginning of the nineteenth 

 century. And the years which have elapsed since Dalton' s time 

 have only served to confirm the essential truth of his theory, 

 though recent investigations and discoveries have tended 

 materially to modify its details. 



The essence of the " Atomic Theory " as held in recent years 

 is that substances are not homogeneous and infinitely divisible, 

 but are composed of ultimate particles. Those particles differ 

 in size and character in different substances, and have been 

 called molecules. A molecule may be defined as the smallest 

 portion of a substance which can exist as such, possessing the 

 distinctive properties of that substance. Molecules are built up 

 of still smaller particles called atoms. In simple or what we 

 call elementary bodies such as oxygen, hydrogen, sulphur, and 

 iron, the atoms which compose a molecule are all of one kind. 

 In compound bodies, which are composed of two or more 

 elements, the molecules are composed of two or more different 

 kinds of atoms ; and the molecules of organic compounds are 

 often exceedingly complex, and many of them may contain 

 hundreds of atoms of several different kinds. A simple body or 

 an element is a substance which can not be decomposed into 

 simpler substances by any process at present known to Chemists ; 

 and a compound body is composed of two or more simple bodies. 



