82 Scientific Proceedings, Royal Dublin Society. 
Nature’s Work AT CLosER QUARTERS. 
We can, however, extract from Maxwell’s determination infor- 
mation about still smaller quantities. In fact, Clausius had’ 
previously been able to show’ that in the more perfect gases at 
ordinary temperatures and pressures, the mean length of the 
free path is about sixty times what the average spacing of the 
molecules is at any one instant of time. By combining Clausius’s 
estimate with Maxwell’s determination, the present writer was 
able, in 1860, to infer that the average spacing of the molecules 
of a gas at the temperatures and pressures which prevail in our 
houses is about a ninth-metret, and that accordingly there are 
about a uno-eighteen of molecules (one followed by eighteen 
ciphers) in each cubic millimetre of the gas. This estimate was 
communicated to the Royal Society in May, 1867, and wiil be found 
in the Phil. Mag. for August, 1868, p. 141. Further, it is known 
to chemists that there are two chemical atoms in each molecule of 
many gases. From this and from the known degree in which 
vapours contract when they are condensed into the liquid or solid 
state, we may infer that the average spacing of chemical atoms in 
solids and liquids lies somewhere in the neighbourhood of the 
tenth-metret (0°000,000,000,1 of a metre), and that accordingly 
there are something lke a uno-twentyone of chemical atoms in 
each cubic millimetre of solids and liquids—not exactly that 
number, but somewere near it. He thus arrived at an estimate— 
an estimate, not a determination—as to the number of molecules 
in a gas, and as to the number of chemical atoms in solids and 
liquids. Such knowledge is imperfect, but is much better than 
knowing nothing about the scale on which Nature is working in 
this branch of her operations. 
The general results of the information acquired in 1860 was :— 
1. That the mean length of the free paths of the mole- 
cules of air at a barometric pressure of 760 millimetres and 
at a temperature of 17° C. is about six eighth-metrets. This 
was a determination. 
* Pogg. Ann. 1858, vol. iii. p. 251; or Phil. Mag. 1859, vol. xvii. p. 89. 
