THE SMITHSONIAN INSTITUTION. 183 



much change in its qiianlily. It varies fronn two per cent, to an inap- 

 precial)le portion. 



Again, the chemical composition of air had been established several 

 years before its mechanical character was fully made out. The sages 

 of Egypt and of Greece disputed about the constitution of matter; but 

 their speculations, however ingenious, led to no definite results. At 

 the end of the last century many valuable facts had been accumu- 

 lated on this point; but no great law had i^een proposed to link these 

 facts together before John Dalton applied the atomic theory of the 

 constitution of matter to explain the mechanical phenomena of the atmo- 

 sphere. 



Dalton did much for chemical science, and is justly regarded as the 

 father of meteorolog3% He was gifted with gigantic powders of mind, 

 and, in other respects, possessed a noble character. 



Modern science regards matter as made up of atoms endowed with 

 attracting and repelling f()rce. In the case of a solid or liquid, these 

 two tierces are in equilibrium — the atoms are held at a distance 

 from each other, and do not fill all the space enclosed within their 

 boundaries. If a solid or liquid is subjected to pressure, the atoms are 

 made to approach each otljer, and the re[)ulsion is increased ; so that, 

 when the pressure is removed, the atoms fly back to their original posi- 

 tion. If, on the contrary, we attempt to draw a solid apart, the attrac- 

 tion comes into operation, and offers a resistance which is called cohe- 

 sion. In the case of aeriform substances, the repulsion entirely pre- 

 ponderates. Dalton gave to this theory a definite form, and applied it 

 to the phenomena of the atmosphere. 



All our conceptions of the constitution of substances, in regard to 

 their solid, liquid, or aeriform states, are more or less intimately asso- 

 ciated with the atomic constitution of matter in its relations to heat. 

 Thus, the action of heat converts a solid into a liquid, by giving mo- 

 bility to its atoms. The action of heat converts a liquid into an elastic 

 vapor, or gas, by imparting a repulsive force to its atoms. Indeed, in 

 regard to gases, the repulsive force and heat are often looked upon as 

 identical, and we shall consider them to be so. The elastic properties 

 of gas, steam, or vapor, are, then, owing to the mutual repulsion of the 

 atoms, in consequence of the action of heat. This view, arising natu- 

 rally out of the atomic constitution of matter, gives an explanation of 

 the mechanism of gases no less simple than consistent. 



To illustrate this principle one fact will suffice. If water is con- 

 verted into steam under the ordinary pressure of the atmosphere, a 

 cubic inch is transformed into about a cubic toot of vapor. The atoms 

 of water are, therefore, twelve times further japart in the case of steam 

 than they are in that of the liquid. The action of heat has had the effect 

 of putting every atom in a state of repulsion with regard to its fellows — 

 every one lends to fly from the other with as much force as if each was 

 under the influence of a powerful spring. The intensity of the repulsion 

 of the atoms constitutes the force of the steam. The elastic properties 

 of the gases of the atmosphere are also owing to the mutual repulsion of 

 their atoms, though the repulsion is much more permanent in this^case 

 than in that of steam. No cold or pressure has yet been found suffi- 

 cient to reduce nitrogen or oxygen to a liquid form. 



