92 



CHEMISTRY. 



tbeir natural explanation indifferences of the re- 

 spective arrangement of like atoms. The term 

 " equivalent '' was introduced to indicate the 

 proportional weights of analogous substances 

 which wore found to be of equal value in 

 their chemical effects. Tables of the equivalent 

 weights of acids were made, representing the 

 proportions of the several substances that were 

 found to be of equal value in neutralizing a 

 fixed quantity of a certain base; and similar 

 tables were made for the bases, as well as for 

 the elements themselves. But little attention 

 was paid under the dualistic system to the es- 

 sential difference between atomic weights and 

 equivalent weights; but under the later devel- 

 opments of the theory of the functions of 

 atoms, it has become 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. 

 From this has sprung the division of the ele- 

 ments into classes consisting respectively of 

 equivalent atoms known as monads, dyads, 

 triads, tetrads, etc., the character of which is 

 well represented in the four typical compounds, 

 Cl H, OH 2 , NH 3 , CH 4 , where chlorine ap- 

 pears as a monad, oxygen as a dyad, nitrogen 

 as a triad, and carbon as a tetrad. This has 

 probably been one of the most important steps 

 yet made in the development of the atomic 

 theory, and has been seen to correspond in so 

 clear and striking a manner with a vast number 

 of well-known properties and reactions of com- 

 pounds as to deserve and acquire the confident 

 trust of chemists. Another great step has re- 

 cently been made which may be destined to 

 lead to most important results. It has been 

 discovered that if we arrange the elements in 

 the empirical order of their respective atomic 

 weights, beginning with hydrogen, and pro- 

 ceeding thence step by step up to the heaviest 

 atom, we shall have before us a natural series 

 with periodically recurrent changes in the 

 chemical and physical functions of its mem- 

 bers. Of course the series is still imperfect, 

 and exhibits gaps and irregularities ; but some 

 of the gaps have been filled up by the discov- 

 ery of elements possessing the anticipated 

 properties, inducing the hope that the others 

 may be filled. The generalization affords a 

 brilliant addition to the previous corrobora- 

 tions of the reality of the units of matter 

 which chemists have discovered. But little 

 account has as yet been taken of atomic mo- 

 tions ; but it has been proved that the heat of 

 combination affords a measure of its force, and 

 we know that, in giving off heat, particles of 

 matter undergo a diminution of the velocity of 

 their motion. The force of chemical combina- 

 tion is evidently a function of atomic motion, 

 but a vast amount of work will be required to 

 develop the atomic theory to the point of ex- 

 plaining the force of chemical action in precise 

 terms of such motion. 



ATOMIC AND MOLECULAR WEIGHTS. Varia- 

 tions in Atomic Value. Professor A. W. Will- 



iamson regards the opinion that atomic values 

 are invariable, or are variable only within par- 

 ticularly defined limits, as an error. He re- 

 marked in a recent address that he had been 

 frequently struck by the fact that two theo- 

 ries, believed at one time to be conflicting, had 

 been shown by the progress of study to be both 

 true. Such was the case with the rival the- 

 ories, one of which represented molecules as 

 constructed after the pattern of three or four 

 types, while the other viewed them as contain- 

 ing complex groups called radicles. Opposi- 

 tion existed at one time between those who 

 made use of atomic weights and those who 

 employed equivalent weights; and the most 

 important step that has of late been taken is 

 the introduction of the notion of equivalence 

 into the atomic theory. An inspection of the 

 series of hydrogen compounds with chlorine, 

 oxygen, nitrogen, and carbon, will show that 

 the atom of chlorine, which combines with a 

 single atom of hydrogen, has a different value 

 from the atoms of oxygen, nitrogen, and carbon, 

 which combine respectively with two, three, 

 and four atoms of hydrogen. Hence, nitrogen 

 and other elements of like equivalence are called 

 trivalent or triads, while other elements are 

 classed, according to the exponents of their 

 equivalence in groups, as monads, dyads, pen- 

 tads, etc. Kekule" still holds that an element can 

 belong to only one of these groups ; that nitro- 

 gen, for instance, is trivalent only, and that in 

 sal-ammoniac, where it is combined with five 

 other atoms, instead of being pentivalent, it is 

 a molecular compound of two chemical com- 

 pounds ammonia and hydric chloride ; and 

 that the atoms of constituent molecules and 

 the molecules themselves were held together 

 by different forces, one being molecular, the 

 other chemical. We have, however, no grounds 

 for assuming a difference between chemical 

 and physical forces, and Kekul6's theory is no 

 longer tenable. The theory commonly in vogue 

 is that atoms vary in value within certain nar- 

 row limits ; that nitrogen, for instance, is either 

 trivalent or pentivalent. Professor William- 

 son recognizes no limitation to atomic value; 

 but he knows that many elements have atomic 

 values greater than those commonly assumed. 

 The character of the atoms often materially 

 affects the result. Thus gold can not combine 

 with more than three atoms of chlorine alone, 

 but it can take up an additional atom of chlo- 

 rine if supplied with an atom of sodium. In 

 this way we get the common double chloride 

 of gold and sodium, NaAuCU, in which the 

 gold is pentivalent. We are not to consider 

 the sodium as being here combined with the 

 gold as such, but as combined with the whole 

 group. Temperature, also, has an influence 

 upon the atomic value of elements, a rise of 

 temperature tending to diminish it. 



Molecular Weight of Hydrofluoric Acid. 

 Professor J. W. Mallet has made some studies 

 of the atomic weight of hydrofluoric acid, 

 with a view to finding an explanation of some 



