14 THE POPULAR SCIENCE MONTHLY. 



If, in the beginning of this century, we had asked, What is a gas ? 

 the answer then would have been that it is matter, expanded and rare- 

 fied to such an extent as to be impalpable, save when set in violent 

 motion ; invisible, incapable of assuming or of being reduced into any 

 definite form like solids, or of forming drops like liquids ; always 

 ready to expand where no resistance is offered, and to contract on 

 being subjected to j)ressure. Sixty years ago such were the chief at- 

 tributes assigned to gases. Modern research, however, has greatly 

 enlarged and modified our views on the constitution of these elastic 

 fluids. Gases are now considered to be composed of an almost infinite 

 number of small particles or molecules, which are constantly moving 

 in every direction with velocities of all conceivable magnitudes. As 

 these molecules are exceedingly numerous, it follows that no molecule 

 can move far in any direction without coming in contact with some 

 other molecule. But if we exhaust the air or gas contained in a closed 

 vessel, the number of molecules becomes diminished, and the distance 

 through which any one of them can move without coming in contact 

 with another is increased, the length of the mean free path being in- 

 versely proportional to the number of molecules present. The further 

 this 23rocess is carried the longer becomes the average distance a mole- 

 cule can travel before entering into collision ; or, in othel- words, the 

 longer its mean free path, the more the physical properties of the gas 

 or air are modified. Thus, at a certain point, the phenomena of the 

 radiometer become possible, and on pushing the rarefaction still fur- 

 ther — i. e., decreasing the number of molecules in a given space and 

 lengthening their mean free path — the experimental results are obtain- 



pher, a considerable degree of belief in the association of the radiant form with the otli^rs 

 in the set of changes I have mentioned. 



As we ascend from the solid to the fluid and gaseous states, physical properties dimin- 

 ish ill number and variety, each state losing some of those which belonged to the preced- 

 ing state. When solids are converted into fluids, all the varieties of hardness and softness 

 are necessarily lost. Crystalline and other shapes are destroyed. Opacity and color 

 frequently give way to a colorless transparency, and a general mobility of particles is 

 conferred. 



Passing onward to the gaseous state, still more of the evident characters of bodies are 

 annihilated. The immense differences in their weight almost disappear ; the remains of 

 difference in color that were left are lost. Transparency becomes universal, and they 

 are all elastic. They now form but one set of substances, and the varieties of density, 

 hardness, opacity, color, elasticity, and form, which render the number of solids and fluids 

 almost infinite, are now supplied by a few slight variations in weight, and some unimpor- 

 tant shades of color. 



To those, therefore, who admit the radiant form of matter, no difficulty exists in the 

 simplicity of the properties it possesses, but rather an argument in their favor. These 

 persons show you a gradual resignation of properties in the matter we can appreciate as 

 the matter ascends in the scale of forms, and they would be surprised if that effect were 

 to cease at the gaseous state. They point out the greater exertions which Nature makes 

 at each step of the change, and think that, consistently, it ought to be greatest in the 

 passage from the gaseous to the radiant form. — (" Life and Letters of Faraday," vol. i., 

 p. 308.) 



