248 AN UNDISCOVERED GAS. 



Ijroperties and on the sequence of their atomic weights, which had at 

 that time only recently been discovered. Thus calcium, strontiam, 

 and barium formed such a group; their oxides, lime, strontia, and 

 baryta are all easily slaked, combining with water to form soluble 

 lime-water, strontia-water, and baryta-water. Their sulphates are all 

 sparingly soluble, and resemblance had been noticed between their 

 respective chlorides and between their nitrates. Eegularity was also 

 displayed by tlieir atomic weights. The numbers then accepted were 20, 

 42.5, and 65; and the atomic weight of strontium, 42.5, is the arith- 

 metical mean of those of the other two elements, for (G5 + 20) / 2 = 42.5. 

 The existence of other similar groups of three was pointed out by 

 Dobereiner, and such groups became known as " Uobereiner's triads." 



Another method of classifying the elements, also depending on their 

 atomic weights, was suggested by Pettenkofer, and afterward elaborated 

 by Kremers, Gladstone, and Cooke. It consisted in seeking for some 

 expression which would represent the differences between the atomic 

 weights of certain allied elements. Thus, the difl'erence between the 

 atomic weight of lithium, 7, and sodium, 23, is 16; and between that of 

 sodium and of potassium, 39, is also 16. The regularity is not always 

 so conspicuous. Dumas, in 1857, contrived a somewhat complicated 

 expression which, to some extent, exhibited regularity in the atomic 

 weights of fluorine, chlorine, bromine, and iodine, and also of nitrogen, 

 phosphorus, arsenic, antimony, and bismuth. 



The upshot of these efforts to discover regularity was that, in 1864, 

 Mr. John Newlands, having arranged the elements in eight groups, 

 found that when placed in the order of their atomic weights, "the 

 eighth element, starting from a given one, is a kind of repetition of the 

 first, like the eighth note of an octave in music." To this regularity 

 he gave the name "The Law of Octaves." 



The development of this idea, as all chemists know, was due to the 

 late Prof. Lothar Meyer, of Tiibingeu, and to Professor Mendeleeff, of 

 St. Petersburg. It is generally kiiowu as the "Periodic Law." One 

 of the simplest methods of showing this arrangement is by means of a 

 cylinder divided into eight segments by lines drawn parallel to its axis; 

 a spiral line is then traced round the cylinder, which will, of course, be 

 cut by these lines eight times at each revolution. Holding the cylinder 

 vertically, the name and atoinic weight of an element is written at each 

 intersection of the spiral with a vertical line, following the numerical 

 order of the atomic weights. It will be found, according to Lothar 

 Meyer and MendeUeff, that the elements grouped down each of the 

 vertical lines form a natural class. They possess similar properties, 

 form similar compounds, and exhibit a graded relationship between 

 their densities, melting x)oiuts, and many of their other properties. 

 One of these vertical columns, however, diifers from tlie others, inasmuch 

 as on it there are three grouj)s, each consisting of three elements with 

 approximately equal atomic weights. The elements in question are 



