8$ Prof. H. E. Armstrong on the Determination of 



(9) Heat of Combustion of Haloid Compounds. — The heat 

 produced by the combination of the halogens may be deduced 

 by subtracting from the observed heat of formation of the 

 haloid compound from gaseous atomic carbon at constant 

 volume (F) that due to the combination of the carbon atoms 

 and of the hydrogen with the carbon atoms ; the values 



to be assigned to v and -g in the case of the haloid com- 

 pounds are ; however, slightly different from those assigned in 



2r 

 the case of the hydrocarbons, viz. — =15720, v = 13960. 



As regards the values for chlorine thus arrived at, it will 

 be seen, on reference to the last column of Table V., that six 

 of the eight compounds containing a single atom of chlorine 

 give values of 13180 to 13750 units ; in the case of allyl 

 chloride and the isomeric monochloropropylene the numbers 

 are somewhat higher, viz. 14560 and 15870 units : hence the 

 mean value is 13827 units*. The mean values for compounds 

 containing 2, 3, and 4 atoms of chlorine are 33090, 47320, 

 and 59050 units respectively. It would appear, therefore, 

 that the fixation of the four atoms of chlorine with which a 

 carbon atom may combine, involves the development of unequal 

 amounts of heat. Thomsen supposes that in the case of two 

 of the atoms the heat developed amounts to about 16500 units, 

 but to only about 13500 units in the case of the other two. 

 Thus: 



PI 



^- 2 = 13500 =13500. Found 13830. 



ni 

 2^- 2 = 2.16500 = 33000. 33390: 



3^= 13500 + 2.16500 = 46500. 47420. 



PI 



4^ = 2 . 13500 + 2 . 16500 = 60000. 59950. 



As regards the bromine compounds, it is only necessary to 

 point out that of the heat developed in the formation of 

 methyl, ethyl, and propyl bromide, the portion attributable to 



at disposal ; and it is probable that the numbers for the two benzenes are 

 inaccurate (compare footnote, § 2) ; therefore no great weight can be 

 attached to these exceptions. 



* This, of course, is not the amount of heat developed in the combina- 

 tion of chlorine atoms with the hydrocarbon radicals, as heat is absorbed 

 to an unknown extent in separating the atoms composing the diatomic 

 chlorine (or bromine or iodine) molecules. 



