BETM-EEN THE ATOMIC WEIGHTS. 239 



which have preceded it, it will be of value to the science. It was originally made, 

 as has already been said, simply for the purpose of teaching, and never would have 

 been published had it not led to the discovery of the numerical relation between the 

 atomic weights. 



On turning to the table which accompanies this memoir, it will be seen that the 

 elements have been grouped into six series. These correspond entirely to the series 

 of homologues of Organic Chemistry. In the group of volatile acids homologues of 

 Formic Acid, for example, we have a series of compounds yielding similar derivatives, 

 and producing similar reactions, and many of whose properties, such as boiling and 

 melting points, specific gravity, etc., vary as we descend in the series according to a 

 determinate law. From Formic Acid, a highly limpid, volatile, and corrosive fluid, the 

 acids become less and less volatile, less and less fluid, less and less corrosive ; first 

 oily, then fat-like, and finally hard, brittle solids, like wax. As is well known, the 

 composition of these acids varies in the same way, and the variation follows a regular 

 law, so that by means of a general symbol we can express the composition of the 

 class. This symbol for the volatile acids may be written (CoHjOg, HO -j- n (C2H2). 



This description of the well-known series of the volatile acids, applies, word for 

 word, nominihus mutantis, to each of the six series of chemical elements. The ele- 

 ments of any one series form similar compounds and produce similar reactions ; more- 

 over, they resemble each other in another respect in which the members of the organic 

 series do not. Their crystalline forms are the same, or, in other words, they are iso- 

 morphous. Although this may be true of the volatile acids, yet it cannot be proved 

 in the present state of our knowledge. Still further, many of their properties vary 

 in a regular manner as we descend in the series. In one case, at least, the law of the 

 variation is known, and can be expressed algebraically, though in most instances it 

 cannot be determined. Finally, as one general symbol will express the composition 

 of a whole organic series, so a simple algebraic formula will express the atomic weight, 

 or, if you may be pleased so to term it, the constitution of a series of elements. 



These points may be illustrated with any of the series in the table ; with the first, 

 for example, which consists of Oxygen, Fluorine, Cyanogen, Chlorine, Bromine, and 

 Iodine. All these elements form similar compounds, as will be seen by inspecting 

 the symbols of their compounds given at the right hand of the list of names, where 

 the similar or homologous compounds are arranged in upright columns. Moreover, 

 they are all isomorphous, as may be seen by referring to the left hand side of the list, 

 where the similar compounds in each upright series are isomorphous, the numbers at 

 the heads of the columns indicating the systems of crystallization, as is described in 



