Affinity and Dissected {Structural) Formula. 405 



&c.) present in x, 1— x, and 1 gramme of the constituents A 

 and B and the compound respectively ; then at 0° and 760 mil- 

 lims. 



F = W 1 +W 2 -W 3 . 



Let h v h 2 , and h 3 be the number of calories required to alter x 

 gramme of A, 1 — x gramme of B, and 1 gramme of compound 

 respectively to the new conditions as to pressure, temperature, 

 &c; then W l + h v W 2 + A 2 , W 3 -M 3 are the total amounts of 

 energy in the two constituents and the compound respectively 

 under the new conditions ; therefore 



F= (W x + h x ) + (W 2 + k 2 ) - (W 3 + h 3 ) = F + h x + h 2 -~h 3 . 



9. Before comparisons can be made of the affinity in classes 

 of compounds exhibiting chemical connexions and relationships, 

 the values of the F's must be reduced to a common standard. 

 A convenient one for this purpose is that where the affinity is cal- 

 culated at a uniform temperature such that all bodies concerned 

 are gaseous, and where the materials occupy the same space as 

 the products; it is also convenient to compare together the 

 affinities, not for equal weights of substances, but for such 

 weights as would under constant conditions of temperature and 

 pressure occupy the same space in the gaseous condition. This 

 latter comparison is readily made by multiplying the affinities per 

 gramme of substance (reduced to a common standard) by the 

 vapour-densities of each compound respectively ; i. e. if F p F^ 

 F 3 , . . . be the reduced affinities per gramme, and 8 V S 2 , 8 3) . . . 

 the vapour-densities, comparisons are instituted between 



6^, S 2 F 2 , S 3 F 3 ,...&c. 



In accordance with the conventions (wholly apart from hypo- 

 theses as to the constitution of matter) on which chemical sym- 

 bolic notation is based, the chemical relations in which various 

 substances stand to one another are to a great extent indicated 

 by their respective dissected formulae. Since each formula indi- 

 cates a numerical value equal to double the vapour-density, it is 

 more convenient to multiply the affinities per gramme by 28 lt 

 2S 2 , 28 3 , . . . &c. The products 28^, 2S 2 F 2 , 2S 3 F 3 , . . . thus ob- 

 tained may be termed the affinities per metrogramme* ; and by 



* One metropneum is the term suggested to indicate the bulk occupied 

 by 1 gramme of hydrogen under the particular circumstances of pressure 

 and temperature that prevail at any given moment ; or 



1 metropneum =11 160 X 07 q" X '— - cubic centimetres 



at a pressure P and temperature t. The term is used instead of the inde- 

 finite phrase " one volume." 

 A metrogramme is the weight in grammes of two metropneums of any 



