74 Prof. H. E. Armstrong on the Determination of 



also add that I have been led to take a special interest in the 

 work, as many of the results appear to me to lend considerable 

 support to my own views of the nature of chemical affinity, 

 serving to confirm the hypothesis that the affinity relations 

 of the negative elements are altogether peculiar, emphasized 

 by me in my Address to the Chemical Section of the British 

 Association at Aberdeen in 1885, and more fully developed 

 in a recent paper on "Electrolytic Conduction in relation to 

 Molecular Composition, Valency and the nature of Chemical 

 Change: being an attempt to apply a theory of c Residual 

 Affinity '" (Proc. Roy. Soc. 1886, vol. xl. pp. 268-291). 



(1) Manner in which Results are stated. — All the heats of 

 combustion of the carbon compounds that will be referred to 

 are calculated on the assumption that the substance burnt is 

 in the state of gas at 18° C, and that the products are gaseous 

 carbon dioxide and liquid water at this temperature ; they are 

 stated in gram-°C. units. The symbol / stands for " heat of 

 combustion," and when prefixed to a symbol or formula it 

 denotes the heat of combustion of the weight in grams corre- 

 sponding to the symbol or formula used. 



The heat of formation of a compound is the difference 

 between its heat of combustion and the heat of formation of 

 the products of combustion ; the heat of formation of carbon 

 dioxide being taken as 96960 units and that of liquid water 

 at 18° as 38360 units. The value thus calculated is the heat 

 of formation at constant pressure ; that at constant volume is 

 deduced by subtracting from it (n — 2) x 290, n being the 

 number of atoms of gaseous constituents which go to form 

 the molecule of the compound. For example,/. CH 4 = 211930 

 units. The heat developed in the combustion of the same 

 quantity of the contained elements would be 96960 + 2 . 68360 

 = 233680 units, the difference between which and f. CH 4 is 

 233680-211930 = 21750 units, being the heat of formation 

 of methane at constant pressure ; as four atoms of its gaseous 

 constituent go to form a molecule of methane, 21750 — 580 = 

 21170 units is the heat of formation of methane at constant 

 volume. 



In previous publications Thomsen has disregarded the 

 molecular composition of the elements, and has used equa- 

 tions such as 



C,0 2 = 96960 units, H 2 ,0 = 38360 units; 



meaning thereby that in the formation of the compound C0 2 

 from the elements carbon and oxygen, or of the compound 

 H 2 from the elements hydrogen and oxygen, the specified 

 amounts of heat are developed, the comma being used by him 



