34 



CLARKE 



mine and gaseous chlorine ; that is, from the elements in their 

 usual molecular conditions. To compare them properly, varia- 

 bility of circumstances must be, so far as possible, eliminated ; 

 but at present this can only be done in part. We can apply the 

 estimates which have been reached in this memoir, for the heat 

 lost in dissociating the halogen elements. We can also allow 

 for the latent heat of vaporization of bromine, 7 300 calories per 

 gramme molecule, and for the latent heat of fusion and vapori- 

 zation of iodine (11.71 + 23.95) x 254 = 9058 calories, and all of 

 these quantities are to be added to the respective measurements. 

 In this way we get an approximation to uniformity of condition, 

 regarding chlorine, bromine and iodine all as gaseous. For 

 each halogen molecule, then, we add : 



For chlorides 

 For bromides 

 For iodides 



13777 = 13777 

 27554 + 7300 = 34854 

 55108 + 9058 = 64164 



It is also best to reduce each measurement to the equivalent of 

 one halogen atom ; and then the comparison assumes the form 

 given in the following table : 



Lithium 



Sodium 



Potassium.. 

 Magnesium 



Calcium 



Strontium.. 

 Barium 



The values now approach one another very closely, and sug- 

 gest an ultimate identity. Each figure represents the heat de- 

 veloped by the union of one, free gaseous halogen gramme 

 atom with a solid metal, to form a salt in aqueous solution. The 

 ideal gaseous condition cannot be realized throughout, but solu- 

 tions give something like an approximation to it. For the other 

 metallic halides, also in aqueous solution, the data are less simple, 

 but they still exhibit regularities. Here are Thomsen's deter- 

 minations. 



