system of Notation to express Chemical Reactions. 279 



reactive force, , ^ * ? = t , T ^ which effects a crossing 

 H*"0*HM Ik*0*H* 



over of half molecules between the water-molecule and the oxide- 

 molecule. If we decide on cohesive affinity, we may retain the 

 name hydrate of potassa. If we decide on chemical reaction, we 

 must find a different name ; but hydrate of potassium will not 

 do, because the water has been decomposed and no longer exists 

 in the half-molecule compound. Potassic water does not seem 

 an improvement, though it manifests the desire of the new school 

 to hold on to the name while rejecting the thing named. 



The atomic theory derived its origin from the quantitative rela- 

 tion of the substitution equivalents in chemical reactions. The 

 idea of equivalent had then to give way to that of fixed atomic 

 weight, or smallest combining ratio. Next, these were found to 

 have a simple relation to vapour- density, also to specific heat. 

 The last step has been the dynamic theory of heat and gases, 

 which represents the law of volumes as the strictly physical 

 sequence of an equilibrium of pressure and vis viva, or tempera- 

 ture, between perfectly elastic particles o rmolecules of different 

 weights, and the law of specific heat in simple solid bodies as the 

 probable sequence of the independent motion of the chemical 

 atoms while maintaining an equilibrium of vis viva. By mea- 

 suring the specific heat of elementary solids, we are enabled to 

 obtain approximately the smallest combining proportion or atom 

 of these solids in terms of a constant unit. By measuring the 

 specific gravity of a gas or vapour, we obtain the weight of the 

 molecule or smallest part that exists in a free or separate form in 

 terms of a constant unit. We thus come to know that the mo- 

 lecule in a certain class of bodies contains only one atom, in 

 another class two atoms, in a third class four atoms, and that 

 the molecule of one body, sulphur, contains six atoms ; but at a 

 very high temperature this large molecule is separated into three 

 smaller molecules, each of which contains two atoms. Mercury 

 and cadmium are known, from measurement of vapour-density, 

 to have one-atomed molecules ; and it is inferred that most of 

 the metals, such as iron, zinc, copper, lead, &c, have also one- 

 atomed molecules, because they also combine with oxygen in the 

 ratio of one specific-heat atom to an atom of oxygen. But 

 there are four, viz. silver, lithium, sodium, and potassium, which 

 Combine with oxygen in the ratio of two specific-heat atoms of 

 metal to one of oxygen, so that their vapour-densities might per- 

 haps show them to have two-atomed molecules. The dual or two- 

 atomed nature of the molecules of the elements oxygen, hydrogen, 

 &c. is of high interest in regard to the dynamic integral of the force 

 'hat unites the 0* with 0*, the H* with H* • e. g. we may 



