752 Sir J. J. Thomson on the Application oj the 



— 33° C. Hence the energy of the liquid chlorine before 



combination is _ 2 -07x "353 x 10 13 ; 



after combination it is the same as that of the sodiunv 

 Hence the change in energy is 



2-07 x 10 13 (2 x •4213--348--353) 



= 2-07xl0 13 x-U2 ergs, 

 or about 70,000 calories. 



The heat of combination for sodium and gaseous chlorine 

 is 9 7 '8 x 10 3 . If we subtract the correction given by Richards- 

 of 18,000 calories for the vaporization of the liquid chlorine, 

 the heat produced when liquid chlorine combines with sodium 

 would be about 80,000 calories ; so that the theoretical and 

 experimental results are in fair agreement. 



An interesting point in connexion with the energy of a 

 crystallized solid compound is whether all the atoms in 

 a compound are equally spaced, or whether some may be 

 regarded as collected in a group and more intimately 

 connected together than the others. Chemists have intro- 

 duced the conception of radicles such as CN, CH 3 , C 2 H 5r 

 CH, which play the same part in chemical reactions as 

 the atoms of monovalent or divalent atoms. The question 

 is whether in a solid crystalline compound containing these 

 radicles their atoms act as a single group, or whether each 

 of their atoms occupies a separate cell. This question could 

 be answered by a determination of the compressibility of the 

 compound. Thus, take the compound KCN as an example : 

 are there separate cells for the atoms of K, C, N — i. e. y 

 are there three cells for each molecule, or are the C and N 

 atoms paired together in a single cell so that each molecule 

 of KCN furnishes two cells instead of three ? The atomic 

 volume of KCN is 42' 8 : if there are three cells the volume 

 of each cell will be 14*3, if there are two it will be 21*4. 

 The compressibility of the substance diminishes with the 

 size of the cell ; the larger cell would give a compressibility 

 not very different from sodium, the smaller one a com- 

 pressibility less than two-thirds of this value. I cannot, 

 however, find any record of the compressibility of KCN. 

 Take ammonium chloride, NH 4 C1, as another example. 

 The atomic volume is 35*2 : if NH 4 forms a group at the 

 centre of one cell each molecule of NH 4 C1 will furnish 

 two cells, each having an atomic volume of 17'6 inter- 

 mediate between the volumes of the cells in solid lithium 

 and sodium and giving a compressibility between the 

 values for those substances ; if, however, each of the 



