TRANSACTIONS OF SECTION B. 575 



were known to have similar functions. It became necessary to study the relation 

 of equivalence between elementary atoms, instead of studying them from the point 

 of view of elements divisible in any proportion. 



It is worth while noticing the general process by which this intellectual chano-e 

 was brought about; for there is a good deal yet to be done in the matter, and our 

 future progress may be guided by experience gained in the past. 



It was essentially one-sided. One consideration was brought into very 

 prominent relie', and it threw a marvellous light on the matter. It gave us a 

 clear view of the natural order among elements : but, like every other strong lio-ht, 

 it fell on one side only. 



The equality of vapour-volumes had been used with great advantage in con- 

 junction with chemical reactions and other evidence as a characteristic of 

 molecules, and the attention of chemists was greatly arrested by the consideration 

 of four tj'pical compounds, which upon the concurrent evidence of very extensive 

 chemical examination and equality of vapour-volumes were known to have 

 respectively a composition corresponding to the formulae CIH, OH2, NII3, CH^. 



It was known that the atom of oxygen in water can be replaced by chlorine, 

 but that two atoms of chlorine are needed for the purpose. The atom of nitroo-en 

 in ammonia requires three atoms of chlorine to replace it, whilst in marsh gas the 

 atom of carbon is replaceable by four atoms of chlorine. Other elements were 

 studied from the point of view of their respective resemblance to these, and arranged 

 in classes, each of which consisted of atoms equivalent to one another. Thus 

 chlorine, bromine, iodine, fluorine, hydrogen, potassium, sodium, lithium, silver, 

 &c., constituted a class of atoms of equal value, and were called monads. Oxygen, 

 sulphur, selenium, tellurium, calcium, strontium, barium, magnesium, zinc, 

 cadmium, mercury, lead, copper, &c., were classed together as dyads, having equal 

 value amongst themselves, but double the atomic value of the members of the 

 first class. So nitrogen, phosphorus, arsenic, antimony, bismuth, with boron, and 

 some other elements, were considered as forming a class of atoms each of which 

 has three times the value of the monads. The class of tetrads contained carbon, 

 silicon, tin, platinum, &c. 



Many apparent exceptions to these atomic values were satisfactorily explained 

 as due to the partial combination of like atoms with one another. Thus in the 

 vast majority of hydrocarbons, such as C^H^, C„H^, C^Hj, &c., the atoms of 

 carbon do not appear to be tetravalent, inasmuch as each of the molecules contains 

 less than four atoms of hydrogen to every one atom of carbon. It was well known, 

 however, that polyvalent atoms can combine partly with one element, partly with 

 another, and also that like atoms can combine with one another. Why then should 

 not two tetravalent atoms like carbon combine respectively with three atoms of a 

 monad, and also combine with one another? The compound must be a single 

 molecule with the properties known to belong to methyle C.^ Hg. Again, if this 

 molecule were deprived of two of its atoms of hydrogen, each of the atoms of 

 carbon must combine further with the other atom of carbon forming H., C C H., ; 

 find a further step in this same direction would give us acetylene H C C H, in which 

 each atom of carbon is combined with the other to the extent of three quarters of 

 its value, and with one atom of hydrogen. An extension of this reasoning led ta 

 the discovery of long chains of atoms of carbon, each atom forming a link, and each 

 of them (short of the ends) being combined with two other atoms of carbon, whilfr 

 it-i saturation is completed by hydrogen. 



Similar partial combinations of like atoms with one another were recoo-nised in 

 many other classes of compounds, and there is strong reason to expect that the 

 application of the principle will be far more widely extended in proportion as our 

 knowledge of the silicates and other complex classes of compounds becomes some- 

 what definite. 



This incorporation of the doctrine of equivalence into the atomic theory by the 

 division of the elements into classes consisting respectively of equivalent atoms, was 

 probably one of the most important general steps as yet made in the development 

 of the atomic theory. It was seen to correspond in so clear and striking a manner 

 with a vast number of well-known properties and reactions of compounds as to- 



