2G4 Mr. M. M. Pattison Muir on Chemical Classification. 



riue^ wliicli are very electro-negative elenientSj forming ^vell- 

 marked oxyacids. In the fourth section, on the ascending 

 portion of the curve are found arsenic, selenium, and bro- 

 mine, also \yell marked electro-negative elements, forming 

 oxyacids. In the fifth section, similarly placed to the pre- 

 ceding are tin, antimony, and iodine, which are again electro- 

 negative, although to a less marked extent than either of the 

 preceding groups of three. In the sixth section lead and 

 bismuth occupy analogous positions to those occupied by 

 -antimony and iodine in section five ; but lead and bismuth are 

 possessed of but ill-defined negative characters. Now the 

 differences between the atomic volumes of phosphorus, sul- 

 phur, and chlorine, especially between those of sulphur and 

 chlorine, are considerable ; but so are the differences between 

 the atomic volumes of arsenic, selenium, and bromine, espe- 

 cially between those of the two latter elements. In this case, 

 then, it Avould appear as if the less-marked negative character 

 were to be traced to the larger atomic weights of the three 

 elements as compared with those of the preceding three. It 

 is also to be remarked that, while the whole of the elements in 

 that period which includes phosphorus, sulphur, and chlorine 

 are divided into but two groups, one of which is positive and 

 the other negative, in the period which includes arsenic, 

 selenium, and bromine there are four groups — first a positive, 

 then a negative, then again a positive, succeeded finally by a 

 negative group. The three elements in question are situated 

 in the last negative group. Very probably the double change 

 influences the properties of the whole of the elements in the 

 complete period. The same considerations may be applied to 

 the case of tin, antimony, and iodine. The differences be- 

 tween the atomic volumes are here smaller than in the two 

 series we have just considered; the atomic weights of the 

 elements are larger ; and the elements form the second group 

 of negative elements within one period. The negative pro- 

 perties of these elements are not so well marked as those of 

 either of the preceding groups of three which we have men- 

 tioned. In the case of lead and bismuth, we have two ele- 

 ments situated somewhat similarly with antimony and iodine ; 

 but the difference between the atomic volumes of the two 

 former is much less than that between the atomic volumes of 

 the two latter ; the atomic weights of lead and bismuth are 

 very large ; these elements occupy a position in the period 

 succeeding the electro-positive elements mercury, thallium, 

 &c., which again succeed the negative elements tantalum and 

 tungsten. Now the negative character of bismuth is ex- 



