592 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Thus, all the variations in the zinc and cadmium series seem to be not 

 only explicable but necessary. 



In order to make the statement of possible types more complete, it is 

 worth while to cite the circumstances attending the formation of an 

 easily volatile eudothermic compound from two elements less volatile- 

 Here we should expect the compound to occupy much more space than 

 the elements, since both tlie chemical and physical internal-pressures are 

 less after combination than before. Such a case is that of carbon di- 

 siilphide, whose formation involves the absorption of 93 kilojoules of 

 heat, accompanied by an increase of volume of 25 milliliters (from 35 to 

 60 milliliters) per gram molecule. This very large increase of volume is 

 too irreat to be referred to the decrease in either the chemical or the 

 physical pressure alone ; its magnitude, therefore, shows that both these 

 causes are working together to cause a change of volume, as the hypoth- 

 esis predicts. It must be borne in mind that a given output of energy 

 in the change of physical compression caused by cohesion would iiave 

 a more marked effect on the volume than the same output in the cliange 

 of cliemical affinity, because the cohesive attraction acts on the expanded 

 and easily compressible part of each atom — namely, the outside sur- 

 face of the molecule, which is compressed only by comparatively slight 

 pressure of its cohesion witli other molecules. In short, when the ex- 

 isting pressure is small, the change of volume for a given additional 

 change of pressure will be large, with a given expenditure of energy. 



That exothermic non-volatile sulphides behave in the usual way, and 

 hence that this behavior of carbon disulphide does not depend upon any 

 peculiarity of suli)hnr, is seen by comparing the contraction of 55 

 milliliters which occurs during the formation of potassic sulphide with 

 that of 2 milliliters in the case of argentic sulphide, the respective heats 

 of formation being 423 and 14. 



It is easy to show that these principles generalize and correct a great 

 part of the irregularities in the curve giving the relation between heats 

 of reaction and decrease in volume. The majority of the other irregu- 

 larities which still remain are undoubtedly due to inaccurate data. The 

 pul)lished values of specific gravities are notoriously untrustworthy, often 

 because they have been based on salts only partially dehydrated. For 

 examples, the specific gravities of zincic and cadmic bromides are usually 

 supposed to have the values 3.64 and 3.79, instead of the true values 

 4.22 and 5.20 respectively, — an error of over twenty-five per cent in 

 the latter case. Again, heats of formation are often untrustworthy. 

 For example, the value for carbon tetrachloride is given by Thomsen as 



