58 MORRIS LOEB 



result of their union with the oxygen of the air is precisely the 

 same. The white, powdery oxide of phosphorus forms in each 

 case, and if I take the same quantity of two kinds of phos- 

 phorus, I shall have, in each case, the same quantity of oxide 

 formed. These facts prove that the two substances are identi- 

 cal in composition, although probably differing in the arrange- 

 ment of their atoms. It fact, it is a common operation of the 

 chemical manufacturer to convert yellow phosphorus into 

 red, as it merely requires heating under pressure to effect the 

 conversion. 



Here is another element, sulphur, which shows the same 

 tendency to appear under different guises representing differ- 

 ent arrangements of the atoms. If I melt sulphur, it becomes 

 a thin liquid; but on heating further it thickens and cannot 

 be poured, even upon the inversion of the test-tube. Yet more 

 heating converts it yet again to a liquid; and if I now pour 

 it into water so as to cool it suddenly, I obtain the sulphur in 

 a very remarkable condition, appearing in many respects 

 like a gum; but it is still sulphur. This might be proved by 

 drying it and setting it aside in a vessel entirely free from 

 all other chemical agencies, when it would in time go back 

 into the condition of the yellow sulphur. By kneading it 

 in my hands, I am able to effect this conversion much more 

 speedily. 



These and various other cases of change happening to a 

 simple substance, separated from other substances, force us to 

 assume that the actual element has undergone no change, but 

 that its atoms have rearranged themselves into new patterns. 

 Such changes are best suited to make clear to you why we 

 make the distinction between the molecules and the atoms. 

 Every substance that we know of is supposed then to be 

 made of molecules built up of one or more atoms. If the 

 molecules are broken up, or their atoms rearranged, we ob- 



