CARBONIC ACID. 71 



crease, there is only the power of plants already 

 mentioned of decomposing it, by abstracting its 

 carbon and setting free the oxygen again (106, 698, 

 710, 745). 



124. When two substances combine together and 

 form a compound, they unite in definite and invari- 

 able proportions. A given weight of carbon, in burn- 

 ing, always combines with a uniform quantity of 

 oxygen, to produce a certain weight of carbonic acid 

 gas ; and this rule holds good in all cases of chemi- 

 cal combination, for it is one of the distinctions 

 between mixture and combination, that we are able to 

 mix two substances together, in any relative propor- 

 tion we like; but we are only able to make sub- 

 stances combine in certain fixed proportions. 



125. Compounds do not always consist of equal 

 parts of their elements, for they can consist of one 

 part of one element, and one, two, three, or more parts 

 of another element; and, indeed, there can frequently 

 be formed several difi'erent compounds, by the union 

 of two elements in various proportions. When, how- 

 ever, we mix together two substances which can unite 

 together, they always combine in one of these fixed 

 proportions; and if there is more of the one element 

 than is requisite to form the compound, it is left un- 

 altered. 



126. Thus we know that every six grains of car- 

 bon, or pure charcoal, require sixteen grains of oxy- 

 gen to burn them perfectly, and convert them into 

 carbonic acid ; and exactly the same quantity will be 



