MOLECULES AND ATOMS 323 



quantity of a substance which enters into chemical reaction with other < 

 molecules, and occupies in a state of vapour the same volume as two 

 parts by weight of hydrogen. 



The molecular weight (which has been indicated by M) of a sub 

 stance is determined by its composition, transformations, and vapour 

 density. 



The molecule is not divisible by the mechanical and physical 

 changes of substances, but in chemical reaction it is either altered in 

 Us properties, or quantity, or structure, or in the nature of the motion 

 of its parts. 



An agglomeration of molecules, which are alike in all chemical 

 respects, makes up the masses of homogeneous substances, in all 

 states. 19 



Molecules consist of atoms in a certain state, of distribution and 

 motion, just as the solar system 20 is made up of inseparable parts 

 (the sun, planets, satellites, comets, <fec.) The greater the number of 

 atoms in a molecule, the more complex is the resultant substance, 

 The equilibrium between the dissimilar atoms may be more or less 

 stable, and may for this reason give more or less stable substances. 

 Physical and mechanical transformations alter the velocity of the 



19 A bubble of gas, a drop of liquid, or the smallest crystal, presents an agglomera- 

 tion of a number of molecules, in a state of continual motion (like the stars of the Milky 

 Way), distributing themselves evenly or forming new systems. If the aggregation of all 

 kinds of heterogeneous molecules be possible in a gaseous state, where the molecules 

 are considerably removed from each other, then in a liquid state, where they are already 

 close together, such an aggregation becomes possible only in the sense of the mutual 

 reaction between them which results from their chemical attraction, and especially in 

 the aptitude of heterogeneous molecules for combining together. Solutions arid othe* 

 so-called indefinite chemical compounds should be regarded in this light. According 

 to the principles developed in this work we should regard them as containing both 

 the compounds of the heterogeneous molecules themselves and the products of their 

 decomposition, as in peroxide of nitrogen, N.^O.j and NO.>. And we must consider that 

 those molecules A, which at a given moment are combined with B in AB, will in 

 the following moment become free in order to again enter into a combined form. The 

 laws of chemical equilibrium proper to dissociated systems cannot ,be regarded in any 

 other light. 



20 This strengthens the fundamental idea of the unity and harmony of type of all 

 creation and is one of those ideas which impress themselves on man in all ages, and 

 give rise to a hope of arriving in time, by means of a laborious series of discoveries, 

 observations, experiments, laws, hypotheses, and theories, at a comprehension of the 

 internal and invisible structure of concrete substances with that same degree of clearness 

 and exactitude which has been attained in the* visible structure of the heavenly bodies. 

 It is not many years ago since the law of Avogadro-Gerhardt took root in science. It is 

 within the memory of many living scientific men, and of mine amongst others. It is not 

 surprising, therefore, that as yet little progress has been made in the province o! 

 molecular mechanics ; but the theory of gases alone, which is intimately connected with 

 the conception of molecules, shows by its success that the time is approaching wheu our 

 knowledge of the internal structure of matter will be defined and established. 



