No. 7.] WURTZ— THEORY OF AT03IS. 383 



of combination, the diversity of which it is necessary to refer to 

 the nature of the elements themselves. The latter impress on 

 each of these compound types a particular character and a special 

 form. The atoms of chlorine are so formed that to one of them 

 only a single atom of hydrogen needs to be added to form hydro- 

 chloric acid ; then that an atom of oxj'gen takes two atoms of 

 hydrogen to form water; that an atom of nitrogen requires three 

 to constitute ammonium, and that an atom of carbon demands four 

 to become marsh-gas. What a difference in the power of com- 

 bination of these elements, and, so to speak, in their appetites 

 for hydro,^en ! And will this difference not be connected with 

 some peculiarities in their mode of existence, to some property 

 inherent in matter itself, and which will impress on each of these 

 hydrogenic compounds a special form ? Such is the case. 



It is now admitted that atoms are not motionless, even in 

 bodies apparently the most fixed and in completely formed com- 

 binations. At the moment when these are being formed the 

 atoms come into violent collision with each other. In this con- 

 flict a disengagement of heat is ordinarily observed, resulting 

 from the expenditure of active energy which the atoms have lost 

 in the melee, and the intensity of this heat-phenomenon gives the 

 measure of the energy of the affinities which have presided at 

 the combination. But there is another thing in chemical pheno- 

 mena besides the intensity of the forces at work, and which are 

 more or less exhausted by a disengagement of heat ; I refer to 

 their mode ; it was of this elective attraction that Bergman spoke 

 a century ago, and which governs the form of the combinations. 

 The atoms of the various simple bodies are not endowed with the 

 same aptitude for combination with each other ; they are not 

 equivalent to each other. This is what is called atomicity, and 

 the fundamental property of atoms is W'ithout doubt connected 

 with the various modes of motion by which they are animated. 

 When these atoms combine with each other, their movements 

 require to be reciprocally co-ordinated, and this co-ordination 

 determines the form of the new systems of equilibrium which 

 will be formed ; that is, the new combinations. 



It is with atoms thus endowed that chemists now construct 

 molecular edifices. Resting at once upon the data of analysis 

 and on the investigation of reactions, they express the composi- 

 tion of bodies by formulae which mark the nature, the number, 

 nnd the arrangement of the atoms which each molecule of these 



