1895.] Argon, a new Constituent of the Atmosphere. 285 



and a definite critical temperature and pressure ; and because on 

 compressing the gas in presence of its liquid, pressure remains sen- 

 sibly constant until all gas has condensed to liquid. The latter 

 experiments are the well-known criteria of a pure substance ; the 

 former is not known with certainty to be characteristic of a mixture. 

 The conclusions which follow are, however, so startling, that in our 

 future experimental work we shall endeavour to decide the question 

 by other means. 



For the present, however, the balance of evidence seems to point 

 to simplicity. We have therefore to discuss the relations to other 

 elements of an element of atomic weight 40. We inclined for long 

 to the view that argon was possibly one or more than one of the 

 elements which might be expected to follow fluorine in the periodic 

 classification of the elements elements which should have an atomic 

 weight between 19, that of fluorine, and 23, that of sodium. But 

 this view is apparently put out of court by the discovery of the mon- 

 atomic nature of its molecules. 



The series of elements possessing atomic weights near 40 are : 



Chlorine 35'5 



Potassium 39*1 



Calcium 40'0 



Scandium 44' 



There can be no doubt that potassium r calcium, and scandium 

 follow legitimately their predecessors in the vertical columns, lithium, 

 beryllium, and boron, and that they are in almost certain relation 

 with rubidium, strontium, and (but not so certainly) yttrium. If 

 argon be a single element, then there is reason to doubt whether 

 the periodic classification of the elements is complete ; whether, 

 in fact, elements may not exist which cannot be fitted among 

 those of which it is composed. On the other hand, if argon be a 

 mixture of two elements^ they might find place in the eighth 

 group, one after chlorine and one after bromine. Assuming 37 (the 

 approximate mean between the atomic weights of chlorine and 

 potassium) to be the atomic weight of the lighter element, and 40 

 the mean atomic weight found, and supposing that the second element 

 has an atomic weight between those of bromine, 80, and rubidium, 

 85'5, viz., 82, the mixture should consist of 93'3 per cent, of the lighter, 

 and 6*7 percent, of the heavier element. But it appears improbable 

 tha.t such a high percentage as 6' 7 of a heavier element should have 

 escaped detection during liquefaction. 



If it be supposed that argon belongs to the eighth group, then its 

 properties would fit fairly well with what might be anticipated. For 

 the series, which contains 



C: IV plllandV CJTI to VI Q ~,q pi ItoVII 



oi ;i 4 > ana uig 



