Combination of the Elements and their Atomic Weights. 43 



The curve thus produced may be divided into three principal 

 series, with subordinate series between these. The curve is 

 not complete, many of the elements theoretically required to 

 form the fourth and fifth series not having yet been discovered. 



The first four principal maxima correspond to every other 

 element in Mendelejeff's first column — namely, lithium, potas- 

 sium, rubidium, caesium. Of the other elements in Mendele- 

 jefF's first column, sodium occupies the first subordinate 

 maximum, and copper and silver subordinate minima. 



I enclose a copy of the curves for chlorine, bromine, and 

 iodine. The numbers for the heats of combination are taken 

 from the table in Naumann's Thermochemie (page 451), con- 

 taining values for about thirty elements. Wherever possible, 

 a compound containing two atoms of chlorine was chosen, and 

 all other compounds were expressed in terms of two atoms of 

 chlorine. Thus the number for KC1 was multiplied by two, 

 the number for Al 2 Cl 6 was divided by three, &c. The num- 

 bers on the diagram are therefore the atomic heats of combi- 

 nation of the elements with equal weights of chlorine. The 

 numbers for the heats of combination in presence of water 

 have been preferred. 



The heats of combination of rubidium, caesium, gallium, and 

 indium with chlorine have not yet been determined; the curve 

 is therefore represented by broken lines above strontium, 

 barium, zinc, and cadmium. 



Small negative values are given in Naumann's table for 

 some of the minima placed along the zero-line of the curve. 

 The heat of combination of chlorine with chlorine is, of course, 

 zero in terms of the other numbers. 



The curve is drawn through among the observations, and 

 not from point to point. This shows the curve better; and is 

 quite justifiable when the inaccuracy of the heat-values (due 

 to physical changes) is considered. The curves for bromine 

 and iodine compounds agree very closely with the chlorine 

 curve. An oxygen curve is very irregular, and does not seem 

 to follow any law. Any one who will examine the tables for 

 the heats of combination with oxygen will see how difficult 

 it is to get numbers that have been taken under the same 

 physical conditions. 



In the case of chlorine compounds, by taking the numbers 

 expressing the heats of combination in presence of water we 

 get most of the elements combining under similar conditions. 

 The curve plotted from these values is more regular and sym- 

 metrical than the curve plotted from the values usually called 

 the heats of combination with chlorine. 



I can hardly believe the relations shown in these curves to 



