124 CHEMICAL DISCOVERY AND INVENTION 



intimately associated with a definite and limited number of 

 another kind. Later investigations showed that in every case 

 the associated atoms occupy in space relative positions toward 

 one another which are all definite, and that the properties of the 

 body are connected with and dependent on the configuration of 

 the resulting mass, which is called a molecule. The study of 

 these relationships constitute the department of science known 

 as stereo-chemistry, or chemistry in space. It has led to many 

 discoveries in later times, and is still, at the present day, a very 

 important field of investigation. 



Dalton himself began attempts to determine the relative 

 masses of the atoms conceived by his hypothesis, but the 

 experimental methods available in his time did not admit of the 

 attainment of accuracy. The subject, however, was pursued 

 with improved methods and greater skill by a number of the 

 most able chemists in the former half of the nineteenth century. 

 The names of Berzelius, Dumas, Gay-Lussac, and Stas are 

 prominent among the workers in that field. 



The result of all their labours and that of a host of others was 

 the establishment of a list of substances recognised as elements, 

 in the sense already defined, together with the numbers which 

 represent the relative masses of their atoms or what are called 

 their atomic weights. The revision and criticism of these 

 numbers has for many years past been undertaken by an Inter- 

 national Committee of chemists, and a list is issued annually y 

 under the authority of this committee, which represents the 

 latest and best estimates of these important values. Till a few 

 years ago hydrogen, as the lightest body known, was used as the 

 standard for comparison, but after much discussion in the 

 chemical world, which it is not necessary to follow in this place, 

 it appeared more convenient to assume the atomic weight of 

 oxygen to be represented exactly by the whole number 16, and 

 to calculate all the rest accordingly. Hence the atomic weight 

 of hydrogen is not now represented by the number 1*0, which 

 would require the atomic weight of oxgyen to be 15-88, but by 

 the figure 1-008 as given in the following table. 



It ought to be understood that the atomic weights given in 

 the list are not all equally trustworthy. Some, such as chlorine, 

 potassium, silver, barium, represent a very high degree of 

 accuracy, while others, for various reasons, such as possible 

 presence of impurities or difficulties in manipulation of the com- 



