TRANSACTIONS OF THE SECTIONS. 67 



reduced is exactly the reverse — copper with the greatest atomic weight being the 

 most reducible, nickel and cobalt, iron and manganese following in order. 



In group 2, zinc being the most active, the least reducible, and, on the contrary, 

 platinum the most inactive, the most reducible, another illustration of the law is 

 afforded, — the striking differences between the weight of the atoms finding its repre- 

 sentative in the equally striking differences between the properties of the respective 

 metals. Similar results are afforded by the study of other groups ; thus in the 

 chromium, molybdenum, and tungsten group, chromium — the most active, the most 

 difficult to reduce, the metal forming the largest number of combinations, — possesses 

 the least atomic weight of any member of the series, and precisely as the weight of 

 the atom increases, in molybdenum, and then in tungsten, does the chemical activity 

 decrease. 



The group sulphur and selenium offers a pertinent illustration of the existence of 

 the law ; selenium having an atomic weight 2J times greater than that of sulphur, 

 being to a corresponding extent the more reducible of the two. 



Lead, gold, and silver afford another illustration of the universality of the law, — 

 lead having the least atomic weight, being the most active, the least reducible, and 

 gold the greatest atomic weight, being the least active, the most reducible. 



In the group chlorine, bromine, and iodine, the relation is very evident (the atomic 

 volume of chlorine being that of the liquid state). Phosphorus, antimony and 

 bismuth, and many other groups, when studied in a similar manner, confirm the 

 generality of the law, as applied to groups of elements having equal or nearly equal 

 atomic volumes. 



These interesting results induced the author to extend the survey, and to institute 

 a careful examination into the general relations existing between the atomic weight, 

 atomic volumes, and properties of the whole of the elements. Considerable details 

 were entered into, the results arrived at being summed up as follows : — 



1. That elements having a small atomic weight and a small atomic volume, such 

 as carbon, aluminium, sulphur, are difficult to reduce from their compounds. When 

 they are isolated, they are endowed with a certain degree of permanence ; but the 

 limit of their resistibility is easily attained. 



2. Elements with a small atomic weight and a large atomic volume, such as 

 potassium, sodium, phosphorus, are invariably active, and difficult to keep in an 

 isolated state. 



3. Elements having a large atomic weight associated with a small atomic volume, 

 such as platinum, iridium, are characterized by their capability of resisting chemical 

 and physical agencies. 



4. Elements possessing alarge atomicweight, associated with alarge atomic volume, 

 such as gold, bismuth, have considerable chemical activity; but the motion of the 

 atoms appears to be impeded by reason of their great weight. 



From an accumulated amount of evidence of this nature, the author came to the 

 conclusion that the cohesive or attractive force of the chemical atom bears some 

 marked relation to (if it is not represented by) the actual weight of the atom, and 

 that it is to the repulsive forces associated with the atom that we must attribute the 

 variations in the relative volumes of the elements. The correctness of this conclusion 

 was further confirmed by reference to the atomic constitution of a numerous series of 

 compounds ; but prior to entering into details, reference was made to those elements 

 which possess the peculiar power of condensing upon their surfaces the molecules of 

 the surrounding medium. 



Attention was called to the fact that elements or compounds possessing a small 

 atomic volume are invariably found to be endowed with this property, provided the 

 atomic weight is sufficiently high. 



Thus carbon, an element remarkable for its power of effecting surface condensa- 

 tion, not only possesses a small atomic volume, but the smallest of any known element. 

 Again, the atomic volumes of zinc and platinum are equal, but their atomic weights 

 differ widely, that of zinc being 33 and of platinum 98 ; thus in this group of elements 

 we find that the one possessing the greatest power of inducing surface condensation 

 is the one endowed with the highest atomic weight. Other comparisons of a similar 

 character, and leading to a similar result, induced the author to believe that " surface 

 condensation is caused by the cohesive attraction of the solid exerted upon the sur- 

 rounding molecules of gas." This idea was put forth some time since bv Dr. Faraday. 



5* 



