874 Transactions of the American Institute. 



The symbols here used are intended to convey to the mind an idea 

 of the relative size of combining parts, which is not so apparent when 

 expressed as follows : 



H 2 + Cl 2 = HC1 + HC1 = 2 HOI. 



From the simplest of molecular types we might proceed to the 

 most complex ; and, throughout, if we consider the combining pro- 

 portion of each simple constituent as either a unit or a multiple of a 

 unit, the composition of each molecule may be expressed by whole 

 numbers. Thus, having as many different kinds of units as there are 

 elements, any true chemical combination may be symbolized by a com- 

 bination of arithmetical ratios. This method, under the light of the 

 atomic theory, clearly reveals the harmonic relations of molecular 

 constituents, which, seen from the stand-point of per centage composi- 

 tion, appear unconnected and discordant. 



It must be admitted that well known bodies have not yet been 

 determined quantitatively ; yet, were they made out, we should not 

 be able to demonstrate, by experiment, the truth of the atomic doc- 

 trine. It still remains a theory, in favor of which there are many 

 facts and phenomena that collectively form an argument not easily to 

 be outweighed. This evidence may be briny summarized as follows : 



1. Atomic Weights. Elements_combine in extremely minute parts, 

 according to the law of definite and multiple proportions. The atomic 

 weight of an element is either its equivalent weight or a multiple of 

 it ; as such multiple cannot be divided by reactions, its weight must 

 conform with the atomic number. Whatever changes of position 

 the combining weight of an element may undergo in a series of mole- 

 cular metamorphoses, that is to say, however many times it may be 

 displaced and replaced in chemical combinations, it invariably retains 

 its characteristic weight. This invariability of weight is an essential 

 property of the atom. 



2. Atomic Volume. Gases unite in equal volumes or multiple 

 volumes. If hydrogen be taken as unity, the density of each ele- 

 mentary gas is identical with the weight of its atom. The atomic 

 volume, determined by dividing the atomic weight of a body by its 

 specific gravity, has been the means of revealing many interesting 

 relations among compounds of similar structure, and among many 

 containing different components and of unlike structure. 



3. Atomic Heat. It has been shown by experiment that quantities 

 of each element, conforming with its atomic number, have the same 

 capacity for heat, excepting only carbon, boron and silicon. These, 

 it is believed, will yet be found to conform to the law that the spe- 



