40 



ATOMIC THEORY. 



" 5. In this part the variable stars are in- 

 timately connected with the galaxy, often fall- 

 ing in the gaps, and constantly on the edges 

 of the gaps, but rarely in the center of the 

 star-sprays from the galaxy. Where the zone 

 crosses the preceding part of the galaxy, it is 

 marked sharply and clearly, and seems uncon- 

 nected with the galaxy. 



6. It is a remarkable thing that all the 

 temporary stars with one or two exceptions 

 have appeared in the region where the galaxy 

 and the variable star zone are both broken into 

 two streams. 



" 7. The exceptions to the zone are chiefly 

 found in the bright and short period variables. 



" 8. The addition to the chart of the stars 

 more strongly suspected variable, and that on 

 competent authority, strengthens the zone very 

 much indeed, and but very slightly the number 

 of exceptions." 



The fact that nearly all variable stars of short 

 period are found in a particular zone has also 

 been remarked" by Professor E. C. Pickering, 

 of the Harvard College Observatory. Pro- 

 fessor Pickering describes this zone as extend- 

 ing 16 on each side of a great circle whose 

 pole is in right ascension 195 and north decli- 

 nation 20. The average distance of thirty-one 

 well-known variables of short period from this 

 great circle is 5 30', while a random distri- 

 bution would give an average distance of 30. 



Gold Medal of the Royal Astronomical So- 

 ciety. At the annual meeting of the Royal 

 Astronomical Society of London, in February, 

 1881, the gold medal of the society was award- 

 ed to Professor Axel Holier, for his researches 

 on Faye's comet. 



ATLANTA EXPOSITION. (See EXPOSI- 

 TION, ATLANTA.) 



ATOMIC THEORY. There have been many 

 attempts to establish a law of numerical rela- 

 tions between the atomic weights of the ele- 

 ments. The discovery of definite ratios between 

 the atomic weights and other quantitative at- 

 tributes, the division of the elements into spe- 

 cific groups distinguished by well-marked prop- 

 erties, and the tendency to doubt their primary 

 character and to regard them as derivative 

 combinations of simpler bodies, give a fresh 

 impetus to speculation in this direction. 



MendelejefFs periodic law, confirmed as it 

 has been by the discovery of gallium and other 

 predicted elements, and by the agreement of 

 many established facts with his scheme of pe- 

 riodic functions, which more exact quantitative 

 determinations have rendered more complete, 

 has been elevated into the rank of an accepted 

 theory. 



The Russian chemist has correlated the ele- 

 ments according to a synthetic law which is 

 the most comprehensive yet established in 

 chemistry, co-ordinating all the physical prop- 

 erties and the chemical affinities of the whole 

 list of simple bodies. Arranging the elements 

 in the order of Jheir atomic weights, their den- 

 sities, and consequently their atomic volumes, 



which depend upon the density, and their com- 

 bining numbers in compounds with other ele- 

 ments, each follow a certain progressive order 

 in successive groups of the elements. Similar 

 properties recur with complete regularity, and 

 follow the same order of progression in the suc- 

 cessive series. The properties are modified as 

 the atomic weights increase ; but the modifica- 

 tions affect entire groups, and do not interrupt 

 the gradual progression within the periods. The 

 elements of the different periods in which the 

 same or similar properties are repeated consti- 

 tute the natural families already established by 

 other chemists upon the ground of their iden- 

 tical combining numbers. The atomic weights 

 of contiguous elements usually differ by only a 

 few units. In cases where there is a consider- 

 able hiatus there is also found a gap in one or 

 more of the natural orders, which should be 

 represented here by members of intermediate 

 atomic weights between those of the preceding 

 and the following periods. Some of the gaps 

 in Mendelejeffs scheme have already been filled 

 by subsequently discovered elements. Gallium 

 corresponds in atomic weight and in properties 

 to one of the predicted elements, as do also the 

 descriptions of scandium and ytterbium. Men- 

 delejeff's periodic law is expressed in general 

 terms in the following predicate : All the prop- 

 erties of elements, and consequently of the 

 compounds which they form, are functions of 

 their atomic weights, to which they stand in 

 periodic relations. In the following table all the 

 known elements are arranged in the order of 

 their atomic weights. The horizontal series 

 gives the successive cycles in which the period 

 of progressive development is completed ; and 

 the vertical series, the natural or homologous 

 orders of elements in which the same properties 

 reappear. 



In the following table, it will be seen, telluri- 

 um is the only substance which is out of place. 

 Possibly a redetermination of its atomic weight 

 will give it in this respect the position between 

 antimony and iodine which its intermediate 

 properties indicate. Iron, manganese, and 

 chromium, which differ very slightly in atomic 

 weight, do not exhibit the close resemblance 

 in behavior and properties which the theory 

 requires; and cobalt and nickel, which have 

 almost identical atomic weights and densities, 

 possess, in some respects, quite dissimilar 

 properties. Other differences as remarkable 

 are shown by potassium and calcium, and other 

 proximate elements. Copper, which has many 

 analogies with mercury, here falls in a different 

 group. The gradations of properties are cer- 

 tainly not uniform and proportionate to the 

 atomic weights in the different series, being 

 excessive, for example, between carbon, nitro- 

 gen, oxygen, and fluorine. 



Besides the density, the malleability, duc- 

 tility, fusibility, volatility, and conductivity to 

 heat and electricity of elements seem, in the 

 same manner, to be subject to periodic varia- 

 tions following the increasing order of then* 





