TRANSACTIONS OF SECTION B. 563 



The contention that chemical equality must be regarded as of as clearly defined 

 a nature as gravimetric equality becomes the more weighty when it is reflected 

 that our very definite views concerning gravimetric equality are due solely to the 

 law of conservation of mass, the evidence for and against which, I may remind 

 you, is just now to be discussed by Lord Rayleigh before the Physical Section, 

 The mass of one pound of sodium remains unchanged when the metal is con- 

 verted into salt, washing soda, or borax ; if this were not the case, gravimetric 

 equality would be just as definite as it is now but physicists would have to argue 

 for its general recognition in much the same way as I am doing now for tha 

 recognition of chemical equality. 



In further justification of this claim of chemical equality to coordinate 

 rank with dynamical equality in the quantification of substances it may ba 

 well to take the fact into consideration that the determination of the former 

 is independent of that of the latter. Overlooking the difficulties of the task, 

 let there be at hand or always procurable unlimited numbers of parcels of 

 the diiferent substances to be experimented upon, each of which, by other 

 means than weighing, such as spatial measurement, can be known to be equal 

 to, or greater or less than, other parcels of the same substances. Suppose, now, 

 that after man}' trials, one of a number of equal portions of sodium hydroxide 

 has been found to be the quantity just necessary to interact with one of a number 

 of portions of hydrochloric acid also equal among themselves. The products of the 

 interaction will be some water and some salt. We can now have placed before us 

 a parcel of sodium hydroxide equal to that previously used, another of hydrochloric 

 acid also equal to that used, and the water and the salt obtained, and then have 

 before us chemically equal quantities of four substances. Let now, by spatial 

 measurement, a number of parcels of water be portioned out, all equal to that of 

 the water obtained, and a number of parcels of salt equal to that of the salt 

 obtained. By a series of trials we find a quantity of silver nitrate just sufficient 

 to interact with the sodium chloride, and having, by supposition, taken this quan- 

 tity of silver nitrate from a lot of other parcels equal to it, we find that one of 

 these is just sufficient to interact with one of the portions of hydrochloric acid 

 equal to that used in producing one of the portions of salt. Further, we find that 

 the salt and the hydrochloric acid each produce a substance which is the same, 

 namely, silver chloride, and in the same quantity as the other. Along with it in 

 the case of the salt, is sodium nitrate, and in the case of the hydrochloric acid, 

 nitric acid. We can then find that this quantity of nitric acid is just enough to 

 interact with one of those of sodium hydroxide, and thereby produce quanti- 

 ties of sodium nitrate and water, respectively equal to those obtained in the other 

 interactions. If now we conjoin with these experiments others in which 

 hydrogen, sodium, and silver are each caused to combine with chlorine, and 

 others in which hydrochloric acid, silver chloride, and sodium chloride are electro- 

 lysed into these elementary substances, evidence is obtained of such facts of chemi- 

 cal composition and decomposition and of double decomposition (or what happens 

 when compounds interact) as those upon which the science of chemistry is framed. 



In teaching chemistry the point is kept too much in the background, if not 

 altogether out of sight, that the chemical equality of quantities of different sub- 

 stances is independent of all other relations of equality between them, and that, 

 therefore, its validity is not afl^ected by the fact of its terms agreeing with some 

 and not with other terms of equalities determined in other ways. Instead of 

 bringing out this point the molecule of water is given out as being, primarily and 

 prominently, that quantity which has eighteen units of mass and which measures 

 two unit volumes. Both statements happen in the nature of things to be true, but 

 neither of them describes the molecule. Let it be clearly understood from 

 illustrative examples what is meant by ' chemically equal,' and there is hardly 

 more to be said as to what constitutes a molecule of water than that it is the 

 quantity of it chemically equal to that of some other substance presenting itself 

 for com^parison. ' Molecule ' is a term of relation : it stands for an equal quantity, 

 pot for any particular quantity ; but as such it is as easy to understand and as 

 indefinable as an equal volume or an equal weight of a substance. 



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