748 THE POPULAR SCIENCE MONTHLY. 



hydrogen in the molecules of water, and, measured by the heat 

 evolved, this force is nearly equaled by the force which unites 

 the similar atoms of nitrogen to form a molecule of nitrogen gas ; 

 and the great violence of many modern explosives depends upon 

 this circumstance. 



It will now be seen that with our new philosojjhy the whole 

 glamour which formerly bedazzled our idea of an elementary siib- 

 stance, and distinguished it widely from all other substances, dis- 

 appears. The differences between substances depend upon the 

 differences between their molecules, and as great molecular differ- 

 ences may arise from the union of similar as from the union of 

 dissimilar atoms. The union of two atoms of hydrogen and one 

 of oxygen gives a molecule of water, the union of two atoms of 

 hydrogen and two of oxygen gives a molecule of peroxide of 

 hydrogen ; the union of two atoms of oxygen alone gives a mole- 

 cule of oxygen gas, the union of three atoms of oxygen a molecule 

 of ozone, and the difference between the last two substances is as 

 great and of the same sort as the difference between the first two ; 

 and so it is with the so-called allotropic states of other elementary 

 substances. 



According to the modern philosophy of chemistry, the proper- 

 ties and relations of a substance depend fully as much upon the 

 manner in which the atoms are grouped in the molecules of the 

 substance as upon the nature of the atoms of which the molecules 

 consist ; and the differences between isomeric substances to which 

 we have referred, depend wholly on what we call the molecular 

 structure. The molecules, both of butyric acid and of acetic ether, 

 consist of four atoms of carbon, eight atoms of hydrogen, and two 

 of oxygen, and the chemist will show you just how these atoms 

 are grouped in the molecule of each substance, and how the sepa- 

 rate relations of these widely differing products depend on the 

 structure he has assigned to their respective molecules. Indeed, 

 the study of molecular structure — that is, of the mode of grouping 

 of atoms in the molecules, especially in those of the compounds of 

 carbon — has almost engrossed the attention of chemists for the 

 past twenty-five years. An immense mass of facts and theories 

 has been collected, and a symbolical method of representing the 

 structure has been adopted, which, although highly conventional, 

 must embody real truth, however dimly it may be now perceived ; 

 for the system has led to more, and more important, discoveries 

 than any one of the dominant systems of science of the present 

 day. The system has a great charm for students, and what is 

 called the study of organic chemistry in our colleges is wholly a 

 discussion of problems of this kind. 



These systems of atoms that we call molecules have been fre- 

 quently compared to the solar system, and cited as evidence that 



