746 THE POPULAR SCIENCE MONTHLY. 



mond, graphite, and charcoal. Sulphur, in like manner, was 

 found to crystallize in two wholly incompatible forms, and under 

 these different phases showing such marked differences of quali- 

 ties that they must be regarded as distinct substances. In 1845 

 Schrotter proved that what had before been known as red phos- 

 phorus, and thought to be a lower oxide of the element, was in 

 fact a different condition, an allotropic form, as it was then called, 

 of pure phosphorus — a form which differs as widely from the 

 wax-like, highly combustible material that is so well known as 

 any two substances well could differ. A few years earlier Schon- 

 bein had discovered a new condition of oxygen, which he called 

 ozone, differing widely from ordinary oxygen gas. Now, since all 

 the forms of the same element yield the same products, and hence 

 give the same chemical reactions, it became obvious, as such facts 

 multiplied, that we may have different substances consisting 

 wholly of the same chemical element ; and hence that the chem- 

 ical element, whatever it might be, could not be a definite sub- 

 stance, as Lavoisier had defined it. 



Meanwhile another class of facts became prominent, chiefly in 

 consequence of the investigations in organic chemistry to which 

 Liebig had given such great impulse in Germany. Groups of 

 compounds, consisting for the most part of carbon, hydrogen, and 

 oxygen, came to be known, which, although having exactly the 

 same composition (that is, formed by the union of the same ele- 

 ments in the same definite proportions), had, nevertheless, utterly 

 different properties and relations. Such compounds are said to 

 be isomeric, and a good example may be found in acetic ether, 

 a very fragrant neutral spirit, and butyric acid, whose offensive 

 odor and acrid taste are only too well known in rancid butter. 

 But if oxygen is the acidifying principle of butyric acid, why 

 does it not produce the same effect as an equal constituent of 

 the ether ? Similar phenomena of isomerism soon became very 

 prominent, and forced on chemists the conviction, often against 

 their prejudices, that the nature of the product depended not 

 solely on the nature and proportions of the elements which en- 

 tered into its composition, but quite as much, and even more, on 

 the manner in which the constituents were combined. 



To this phrase — the manner in which the constituents are 

 combined — no definite meaning was at first attached ; but the old 

 atomic theory, first applied in chemistry by Dalton, was soon so 

 modified as to give a form to the conception, and on the distinc- 

 tion between atoms and molecules then introduced the whole 

 philosophy of modern chemistry rests. 



In the subdivisions of material bodies, the molecules are the 

 smallest masses in which the qualities of a substance inhere. A 

 molecule of sugar or salt is simply a very small lump of sugar or 



