302 DIRECT PROPERTIES OF MOLECULES 110 



This conception reconciles in the simplest fashion the 

 apparent contradictions between the properties of gases, and 

 explains the striking circumstance that the phenomena 

 those of viscosity, for instance remain essentially unaltered 

 even when the gas is very greatly rarefied. For the free path, 

 which a molecule can attain in the fine dust of molecules, 

 remains a small magnitude under all circumstances. It is 

 at once obvious from the numbers given that a cubic centi- 

 metre of gas of ordinary density is almost as good as 

 impenetrable by another molecule of gas ; but even if this 

 gas has been rarefied to about a three-thousandth part of 



o J- 



its normal pressure, and so to a pressure of about mm. 

 of mercury, the number of molecules contained in a cubic 

 centimetre would still yet suffice to thickly cover the six 

 faces of the cube which they fill ; this cubical space there- 

 fore seems to remain almost as impenetrable as before, 

 and we see that the molecular free path will be still very 

 small even now. 



111. Section of Compound Molecules 



If we compare the tabulated values of the sums of the 

 sections for different gases, we easily perceive that a simple 

 law holds in several cases. The value 25100 for hydrochloric 

 acid is very nearly equal to 24400, which is the arith- 

 metical mean of the values 9900 and 38800 for hydrogen 

 and chlorine respectively. Nitric oxide, however, does not 

 follow this rule, since its value for Q, viz. 19200, is greater 

 than the mean (18000) of those for nitrogen and oxygen, 

 viz. 18600 and 17400 ; but in this case the conformity to 

 the law indicated might be hidden by these three numbers 

 differing from each other by scarcely more than the possible 

 errors of their determination. The same law appears in 

 another similar case ; thus the difference of the numbers 

 27000 and 18700 for C0 2 and CO respectively, viz. 8300, 

 is with tolerable exactness the half of the number 17400 

 found for 2 . 



These examples seem to indicate that the section of a 

 molecule is equal to the sum of the sections of the atoms 



