154 POPULAR SCIENCE MONTHLY. 



substances, approaching somewhat our present idea of elements, but 

 even down to Lavoisier's time the old Greek conceptions were not 

 abandoned. Lavoisier's definition of an element deserves to be quoted, 

 since more than a century of chemical progress has failed to improve 

 or in any essential way change it. "An element," says he, in his 'Traite 

 de Chimie,' "is a substance from which no simpler body has as yet been 

 obtained; a body in which no change causes a diminution of weight. 

 Every substance is to be regarded as an element until it is proved to 

 be otherwise." With his conception of an element, Lavoisier intro- 

 duced a new and scientific nomenclature into chemistry, which is to a 

 very considerable extent in use to-day. The views of Lavoisier did not 

 gain immediate recognition, but a decade after his untimely death the 

 new ideas had been very generally adopted. 



With the opening of a new century came the rehabilitation of a 

 theory which had originated back in the misty days of early Greek 

 philosophy, but which was now to be given a new value, because no 

 longer a vague guess, but founded upon experimental evidence. This 

 was the theory of the atomic constitution of matter. According to the 

 Greek conception, if matter were divided into smaller and ever smaller 

 portions, at last a point would be reached where the particles are in- 

 divisible, and such particles are the atoms. Dalton seems first to have 

 hazarded the idea of atoms, almost as a speculation, to account (wrongly) 

 for the various different degrees of solubility of different gases in 

 water, and at this early stage he published a table of familiar sub- 

 f^tances, with the atomic weight of each. A decided confirmation was 

 given to this guess by Dalton's discovery that when different gases 

 combine, it is always in proportions expressed by whole numbers. This 

 could most readily be explained by the theory that these gases were 

 made up of indivisible particles called atoms, Avhose union conditioned 

 the proportion between the uniting masses of gases. This atomic 

 theory was somewhat combated by a few chemists, even Sir Humphry 

 Davy and Mr. Wollaston for a brief time opposing it. It soon, how- 

 ever, made its way, and its general principles have been received by all 

 chemists; for nearly a century it has dominated, or rather has been 

 the foundation of, chemical theory. 



According to this theory, all matter is composed of some seventy 

 different kinds of atoms, each possessed of independent and permanent 

 properties. Lists of the atomic weights of the commoner elements were 

 rapidly published, the most notable being that of Berzclius, in 1815. 



The fact of so many different kinds of ultimate particles of matter 

 was naturally a great blow to those who believed in its unity. Very 

 early there was speculation as to whether any connection existed be- 

 tween the different kinds of atoms. The first step in this direction 

 was what is known as Front's hypothesis, which was first enunciated 



