October 25, 1895.] 



SCIENCE. 



537 



and forming a strongly basic ' alkali.' 

 Beryllium (9) is still metallic, but with 

 boron (11) the metallic, base forming 

 properties have vanished, and it is a weak 

 acid former, while in fluorine (19) we have 

 an element which is gaseous, highly nega- 

 tive, an acid former, and as different from 

 lithium as could well be imagined. The 

 next element, sodium (23.05), does not pos- 

 sess in a still stronger degree the properties 

 shown bj' fluorine, as M'e might expect; on 

 the contrary, it closely resembles lithium. 

 A similar gradual modification again occurs 

 as we proceed, the metallic, basic properties 

 fading out and being gradually replaced 

 by non-metallic, acidic properties. Silicon 

 (28.4) is no longer a metal and chlorine 

 (35.45) is a gas, chemically and physically 

 much resembling fluorine. Beginning with 

 the next element, the alkali metal potassium 

 (39.11), a similar series of gradual transi- 

 tions is gone through, followed by a sudden 

 interruption and reversion, and this is sev- 

 eral times repeated through the whole list, 

 with certain modifications which it is not 

 essential to our purpose to consider. Each 

 set of elements beginning with a metal and 

 ending with a non-metallic acid former is 

 called a period. Those elements which fall 

 into the same vertical column constitute a 

 group or natural family and have a marked 

 resemblance, lithium being followed by 

 sodium, potassium, rubidium and CEesium, 

 the alkali metals, and fluorine by chlorine, 

 bromine and iodine, the halogens, all of 

 which give rise to strong acids. 



This system, while presenting some diifl- 

 culties, just as nearly every system of nat- 

 ural phenomena does, bears abundant evi- 

 dence of being based on some natural cause, 

 of the nature of which we know as yet ab- 

 solutely nothing. Quite a number of blanks 

 or ' missing links ' occur, but it is gener- 

 ally expected that elements will ultimately 

 be discovered which will fit into these places; 

 an expectation which has in several cases 



been realized in the most striking manner. 

 The properties of helium and argon and 

 their atomic weights, as at present roughly 

 determined, do not hold out any promise 

 that these elements will fit into any of the 

 vacancies ; on the contrary, it seems quite 

 quite certain that they will not. 



The abrupt jump from the halogen at the 

 end of each period to the alkali metal be- 

 ginning the next, in contrast with the 

 gradual change elsewhere observed, is one 

 of the most remarkable facts in chemistry. 

 G-iven the existence of a periodicity of prop- 

 erties, we should rather expect the periods 

 to be connected by a series of elements 

 showing gradations in a reverse order, or 

 at least by transitional elements of inter- 

 mediate properties. We should expect to 

 find a continuous curve with maxima and 

 minima rather than a series of disconnected 

 lines. While the law which makes the 

 properties of elements a function of their 

 atomic weights is wholly unknown we can- 

 not assert that this must be so, and it is 

 rather our reliance on the principle of con- 

 tinuity which leads us to feel that it should 

 be so. We may assume that the connect- 

 ing links actually exist though as yet un- 

 discovered. It is on such an hypothesis 

 that the most plausible attempts to classify 

 the new elements have been made. 



Eayleigh and Eamsaj^, at the close of 

 their memorable paper on argon, read be- 

 fore the Royal Society in January, pointed 

 out a possible way of harmonizing their dis- 

 covery with the Periodic Law. They called 

 attention to the variation of valencj'' in the 

 latter half of the second period : 



Element; Silicon. Phosphorus. Sulphur. Chlorine. 



Valency: 4 3-5 2-6 1-7 

 Atomic wt. 2S.4 31 32.06 35.45 



The next known element is potassium, 

 beginning the third period with an atomic 

 weight 39.11. Between this and chlorine, 

 however, we may imagine another element, 



