On the Physical Units of Nature. 57 



litres of hydrogen, indicating that the quarter molecule of 

 phosphorus is its atom. The same is true of arsenic. 



A similar treatment of marsh gas furnishes twelve as the mass 

 of an atom of carbon, although carbon is not sufficiently volatile to 

 enable us to ascertain the relation of its atom to its gaseous molecule. 



By extending this method to all the available cases, we may 

 deduce froin the fundamental jpro-perties of gases a demonstration 

 of a great part of the modern, table of atomic weights, and of the 

 doctrine of atomicity which depends on it. Thus, two bonds* 

 are necessary to connect the group S O4 with the two atoms of 

 hydrogen that are united to it in sulphuric acid, while one bond 

 is sufficient to join the atoms of hydrogen and chlorine in an 

 atom of hydrochloric acid and so in other cases. 



Now the whole of the quantitative fiicts of electrolysis may be 

 summed up in the statement that A definite quantity of 



ELECTRICITY TRAV:EESES THE SOLUTION FOR EACH BOND THAT IS 



SEPARATED. Thus, if a current pass in succession through 

 vessels containing solutions of sulphuric acid and hydrochloric 

 acid, two atoms of hydrochloric acid will be decomposed in the 

 one vessel for each atom of sulphuric acid that is decomposed in 

 the other, but the number of bonds separated, vjill be the same in 

 both vessels. 



It is the quantity of electricity that passes per bond separated 

 that we have now to determine, and this may be done 

 approximately in the following manner. Several inquiries (see 

 Prof. J. Loschmidt ' Zur Grosse der Luffcmolecule ' Academy of 

 Vienna, Oct., 1865; G. Johnstone Stoney on 'The Internal 

 Motions of Gases' Phil. Mag. August, 1868; and Sir William 

 Thomson on ' The Size of Atoms ' Nature, March 31, 1870), have 

 led up to the conclusion that the number of molecules in each 

 cubic millimetre of a gas at atmospheric temperatures and 

 pressures is somewhere about a unit-eighteen (10^*^). Hence the 

 number of molecules in a litre will be about a unit-XXIV. Now, 

 a litre of hydrogen at atmospheric pressures, and temperatures 

 weighs, roughly speaking, a decigramme. Hence the mass of 

 each molecule of hydrogen is a quantity of the same order as a 



* The word bond is here used of the connexions between atoms when they enter into 

 combination. When we employ the term in this sense, which seems its proper signilication, 

 bonds are to be distinguished from the hands or feelers which each atom has, and which 

 by grappling with the hands or feelers of other atoms establish bonds between them. 



SciEN. Pkoc, R.D.S. Vol. hi., Pt. ii. p 



