76 PROGRESS OF SCIENCE IN THE CENTURY. 



curate methods have resulted in an enormous increase 

 in the list of elements in the course of the nineteenth 

 century. Special interest is attached to the recent 

 discovery of argon and helium. 



THEORY OF COMBUSTION AND THE CONSERVATION OF 

 MATTER. 



Theory of Combustion. — Since the science of 

 chemistry has to do with the changes in the nature of 

 substances when they combine or separate, and since 

 burning is one of the most obvious of these changes, 

 it is natural that we should give prominence to the 

 theory of combustion. But there is another reason 

 why we should do so here, namely, that some under- 

 standing of combustion marks the beginning of the 

 century-period with which our brief historical sketch 

 deals. It is hardly too much to say that modern 

 chemistry dates from the time when the burning fire 

 began to be in some measure intelligible, or, what 

 comes almost to the same thing, from the time when, 

 oxygen and carbonic acid gas having been discovered, 

 it became possible to measure the changes which take 

 place in a combustion. 



It is interesting, as we sit by the fireside, to think 

 of the different ways in wiiich the familiar sight 

 has been regarded by successive generations of men, 

 from the time when the four elements were first 

 vaguely imagined to the days of " phlogiston " and 

 " principles of combustion," and thence to the 

 present day, — a long story of changing ideas. But 

 it is sufficient for our purpose here to recall, that it 

 was not until about a century ago that there was 

 anything approaching to a scientific vision of the 

 burning fire. 



