OF NATURAL PHILOSOPHY. 301 



oversights, but of necessity, and from an inherent 

 defect of the theory itself, which thus impeded the 

 progress of the science, as far as a science of expe- 

 riment can be impeded by a false theory, by per- 

 plexing its cultivators with the appearance of 

 contradictions in their experiments where none 

 really subsisted, by destroying all their confidence 

 in the numerical exactness of their own results, 

 and by involving the subject in a mist of vision- 

 ary and hypothetical causes in place of the true 

 acting principles. Thus, in the combustion of any 

 substance which is incapable of flying away in fumes, 

 an increase of weight takes place, the ashes are 

 heavier than the fuel. Whenever this was observed, 

 however, it was passed carelessly over as arising 

 from the escape of phlogiston, or the principle of in- 

 flammability, which was considered as being either 

 the element of fire itself, or in some way combined 

 with it, and thus essentially light. It is now known 

 that the increase of weight is owing to the absorp- 

 tion of, and combination with, a quantity of a peculiar 

 ingredient called oxygen, from the air, a principle 

 essentially heavy. So far as weight is concerned, it 

 makes no difference whether a body having weight 

 enters, or one having levity escapes ; but there is 

 this plain difference in a philosophical point of view, 

 that oxygen is a real producible substance, and 

 phlogiston is no such thing: the former is a rera 

 causa, the latter an hypothetical being, introduced 

 to account for what the other accounts for much 

 better. 



(337.) The third age of chemistry that which 

 may be called emphatically modern chemistry 



