110 CHEMICAL DISCOVERY AND INVENTION 



gravities of the gases. And from the known densities of the 

 elementary gases it follows that many of them consist of mole- 

 cules containing more than one atom, e.g. hydrogen, oxygen, 

 nitrogen, chlorine contain two atoms each. 



The determination of the density of the vapours of a large 

 number of substances which, though not gaseous at common 

 temperatures, are convertible into vapours by heat, provides, 

 therefore, a method very generally applicable to the deter- 

 mination of molecular weights. 



A molecule is now always understood to mean the smallest 

 mass of any substance, elementary or compound, which is capable 

 of existing by itself. 



At the time of the publication of Avogadro's hypothesis, or 

 law as it is often called, Michael Faraday was a youth just twenty 

 years of age, and as yet following his occupation of bookbinder. 

 It was only little more than twenty years later that, pursuing 

 the study of electro-chemical decomposition or electrolysis, he 

 gave the world the two great quantitative laws which are 

 generally known as Faraday's Laws of Electrolysis. They may 

 be stated as follows in his own words, which will be found in his 

 Experimental Researches in Electricity (vol. I, p. 241) : 



I. " The chemical power of a current of electricity is in 

 direct proportion to the absolute quantity of electricity which 

 passes." 



II. " Compound bodies may be separated into two great 

 classes, namely, those which are decomposable by the electric 

 current, and those which are not. ... I propose to call bodies 

 of the decomposable class electrolytes. Then again the substances 

 into which these divide under the influence of the electric 

 current form an exceedingly important general class. They are 

 combining bodies, are directly associated with the fundamental 

 parts of the doctrine of chemical affinity, and have each a 

 definite proportion in which they are always evolved during 

 electrolytic action. I have proposed to call these bodies generally 

 ions, or particularly anions and cations, according as they appear 

 at the anode or cathode, and the numbers representing the pro- 

 portions in which they are evolved electro- chemical equivalents. 

 Thus oxygen, chlorine, iodine, hydrogen, lead, tin are ions ; the 

 three former are anions, hydrogen and the two metals are 

 cations, and 8, 36, 125, 1, 104, 58 are their electro-chemical 

 equivalents nearly." 



