Stoney— The known part of Nature's Work. 83 
2. That the mean spacing of the molecules in a gas at 
the same temperature and pressure is of the same order as! 
a ninth-metret. This was an estimate. 
3. That the mean spacing of the chemical atoms of which 
solids and liquids consist lies somewhere in the neighbour- 
hood of a tenth-metret. This, like the last, was an estimate. 
The tenth-metret, the smallest of the above measures, is the 
ten-thousand-millionth part of a metre. It is about the two- 
thousandth part of the smallest interval which the best microscope 
can detect when most carefully handled. 
Another branch of physical inquiry has introduced us into the 
same region of magnitudes, and has even carried us farther. The 
wave-lengths of visible light range from 38 to 76 eighth-metrets, 
and can, by methods which will be described farther on, be mea- 
sured with such marvellous precision that itis possible to detect 
differences of wave-length which amount to a very small fraction 
of a tenth-metret. 
1 Tn Molecular Physics, where our estimates, and even our determinations, inevitably 
fall far short of attaining exactness, it is very convenient to be able to describe the 
result as being ‘‘of the same order as’’ some specified magnitude. 
To give definiteness to this expression, imagine units where there are ciphers in 
fig. 6. They are a geometrical series, each unit having a value ten times that of the 
unit to its right. Next form the corresponding series with We 10 asits factor. This will 
interpolate a new term between every two consecutive terms of the former series. 
Thus, on either side of the unit so situated in our table as to represent a ninth-metret, 
will be terms one of which will have the value \/10 ninth-metrets, and the other 
1)/ 10 of a ninth-metret. Now, any quantity between these two limits may be 
spoken of as ‘‘of the same order as a ninth-metret.” In accordance with this con- 
vention, 3 ninth-metrets, 2 ninth-metrets, 1 ninth-metret, } ninth-metret, and } ninth- 
metret are all quantities ‘‘ of the same order as’’ a ninth-metret. 
When we deduce the number of molecules in a gas from the spacing of the mole- 
cules we have to deal with the cube of an already estimated number, and accordingly 
the range implied by the phrase ‘‘of the same order as’’ becomes widened. It now 
ranges from / 1000 times the assigned value (in this case a uno-eighteen per cubic 
millim) to 1 WA 1000 times this value ; so that it includes 30, 20, 10 times, and 1/10, 
1/20, and 1/30 of a uno-eighteen. The knowledge thus reached as to the number of 
molecules that are present may seem very indefinite; but it is far from being 
valueless. 
G2 
