WHAT IS A MOLECULE f 689 



No one has ever seen or handled a single molecule, and molecular 

 science therefore deals with things invisible and imperceptible by our 

 senses. We can not magnify a drop of water sufficiently to see its 

 structure ; and the theory that matter is built up of molecules depends, 

 like th^philosophy of every science, on its competence to explain ob- 

 served facts. These are of two kinds namely, physical and chemical. 

 A physical change in the condition of a body is illustrated by dissolv- 

 ing a lump of sugar in water. The sugar disappears, but remains 

 present in the water, from which it may be recovered by evaporation. 

 But if we burn the lump, we effect a chemical change in its condition. 

 The sugar again disappears, and in its place we get two other sub- 

 stances namely, carbon and water. 



Similarly, water is converted by boiling into the invisible vapor, 

 steam ; but the change in its condition is physical only, for the steam 

 condenses to water on being cooled. If, however, we pass water 

 through a red-hot iron tube, it disappears, and is replaced by the two 

 gases, oxygen and hydrogen. In the latter case, the liquid suffers a 

 chemical change, or, as we say, is " decomposed " into its constituent 

 elements. Those changes, therefore, which bodies undergo without 

 alteration of substance are called physical ; while those which are ac- 

 companied by alteration of substance are called chemical. 



Turning our attention first to the physical side of the question, let 

 us inquire how far some of the fundamental laws of science are illus- 

 trated by the molecular hypothesis. Among the most important of 

 these is the law of Boyle, which declares that the pressure of gases is 

 proportional to their density. The theory under review is based at 

 present on the phenomena of gases, and considers these as aggrega- 

 tions of molecules in constant motion. Their movements are supposed 

 to take place in straight lines, the molecules hurrying to and fro across 

 the containing vessel, striking its sides, or coming into contact with 

 their neighbors, and rebounding after every collision, like a swarm of 

 bees in a hive flying hither and thither in all directions. 



We know that air, or any gas, confined in a vessel, presses against 

 its sides, and against the surface of any body placed within it. This 

 pressure is due to the impact of the flying molecules ; and the con- 

 stant succession of their strokes is, according to this theory, the sole 

 cause of what is called the pressure of air and other gases. As each 

 molecule strikes the side of the vessel the same number of times, and 

 with an impulse of the same magnitude, the pressure in a vessel of 

 given size must be proportionate to the number of molecules that is, 

 to the quantity of gas in it ; and this is a complete explanation of 

 Boyle's law. Let us next suppose that the velocity of the molecules 

 is increased. Then each molecule will strike the side of the containing 

 vessel not only more times per second, but with greater force. Now, 

 an increase in the velocity of the molecules corresponds in theory to 

 a rise of temperature ; and in this way we can explain the increase of 



TOL. XIX. 44 



