270 ATOMIC WEIGHTS 



All that is assumed in this theory is, that the atoms arc of constant value 

 the same atoms may bo arranged in a different way, and hence, although any par- 

 ticular compound contains always the same elements in tho atomic ratios, yet the same 

 atoms may, by difference in arrangement, give rise to bodies agreeing in composition 

 by weight, but differing essentially in properties. See ISOMEIUSM. 



The atomic theory is further confirmed by the observation, that if tho specific heat 

 of the elements be compared, it is found that in a largo number of cases tho specific 

 heats of quantities of the bodies represented by their atomic weights coincide with each 

 other in a remarkable manner. 



For a full examination of this subject, consult ' An Introduction to the Atomic 

 Theory,' by Charles Daubeny, M.D. ; and ' Memoirs of John Dalton and History of 

 the Atomic Theory,' by Robert Angus Smith, Ph. D. 1 



ATOMIC VOLUMES. Of late years it has been assumed that tho elements, 

 when in the gaseous state, unite invariably in equal volumes, or, in other words, that 

 the atoms of bodies have always tho same volume. If this doctrine be maintained, it 

 becomes necessary to alter tho atomic weights or combining numbers of certain 

 elements. For example, water contains two volumes of hydrogen to one of oxygon ; 

 but, according to the old idea, it consists of singlo atoms of each element; it is clear, 

 therefore, that if we are to assume that the atoms of hydrogen and oxygen have the 

 same volume, wo must either halve the atomic weight of hydrogen or double that of 

 oxygen. 



Berzelius suggested that all the atomic weights should remain the same, except 

 those of hydrogen, nitrogen, phosphorus, chlorine, bromine, and iodine, which should 

 halve their present values. Gerhardt, on the other hand, adopted tho more convenient 

 practice of allowing hydrogen and its congeners to retain their present atomic weights, 

 doubling those of oxygen, sulphur, tellurium, and carbon. This practice has of Into 

 years been extended to many other elements. See ATOMIC WEIGHTS, and Watts's 

 ' Dictionary of Chemistry.' 



ATOMIC WEIGHTS, COMBINING WEIGHTS or PROPORTIONS. 

 The atomic weights of the elements represent the proportions in which they severally 

 combine with each other, referred to some standard element as unity. In accordance 

 with Dalton's atomic theory, explained in a previous article, it is supposed that tho 

 numbers assigned to the elements as their respective combining proportions represent 

 the relative weights of their atoms, and hence the adoption of the term atomic weight 

 a term extremely convenient to retain whatever views may be held as to the 

 ultimate constitution of matter. By Berzelius, oxygen was selected as tho standard 

 to which all atomic weights were referred, but it is now almost universal among 

 chemists to toko hydrogen as the standard of comparison, since it is found that of all 

 the elements hydrogen has the smallest combining, number. It was believed by 

 Prout that the atomic weights of all substances were multiples of the atomic weight 

 of hydrogen, but it has been shown by Stns who has made the most refined experi- 

 ments on this subject that tho theory is only approximately true. 



In establishing the atomic weights, or proportional numbers, of the elements, it is 

 not only necessary to determine exactly the ratios in which they severally combine 

 which is merely a matter of accurate experiment but also to interpret these ratios 

 by the light derived from an extensive range of physical and chemical phenomena 

 such as isomorphism, specific heat, combining volume of vapour, and tho like. 

 Hence, as science has advanced, the necessity has boon recognised by most chemists, 

 of altering the numbers assigned to certain of tho elements as their atomic weights, 

 and, in fact, within tho last few years many of these numbers have been doubled. 

 It has, however, been considered, in the preparation of the now edition of this 

 Dictionary, that the convenience of manufacturers and others accustomed to tho use of 

 the old atomic weights would be best served by retaining these familiar figures ; and 

 hence in all cases throughout this work, unless otherwise stated, tho formula) are 



1 Dr. Angus Smith, in his ' Memoirs of Dalton,' thus sums up tho labours of this deep thinker : 

 ' This Dalton did. He gave the first idea of atomic weights. Under this head came Richter and 

 Fischer's numbers. Bichter, grappling with those numbers, never could obtain a rational theory 

 from the phenomena. Dalton's plan explains these numbers with the greatest ease, and looks 011 

 such as a necessity of the fundamental law, instead of the beginning of the inquiry, as it was to them. 

 It seems to me, then, that what happened historically happened also intellectually. Dalton had 

 included his predecessors in his more extensive system. He had gone to the summit of the hill, and 

 when coming down found proofs that they had been making good progress upwards. Higgins had 

 gone at once to the top, as it appears to me, but took no heed to make the needful observations 

 when he was up, or he found the prospect entirely obscured. We ure compelled to put reciprocal 

 proportions in a secondary position, as it seems to me it cannot be called a law, but one of the con- 

 sequences of a law ; and the evidence brought to support it, otherwise than empirically, presupposes 

 Home of the principles on which the general laws depend. It was by a careful mechanical juxtu. 

 position of parts that Dalton arrived at the idea ; it is eminently mechanical, and it is remarkable that 

 all progressive views on the subject have been so. He introduced proportional weights into the theory , 

 and found it to agree with/acts. His is, therefore, the quantitative atomic theory.' 





