CHEMISTRY. 



149 



standard weight of it (as C 2 , 2 H, &o., are 

 completely saturated by additions respectively 

 of 6H, 5H, &c.) ; and that owing to the diffi- 

 culty of obtaining in some cases hydrogen com- 

 pounds, and to the want of them in others, it 

 has been allowed in practice to take a constant 

 weight of some other element equivalent to a 

 unit-weight of hydrogen, and most frequently 

 so in case of the chlorine, bromine, and iodine 

 compounds, goes on to present certain cases, 

 as that of the union secondarily of the chloride 

 of silver (usually regarded as a perfectly satura- 

 ted compound), with bromide of silver, in the 

 proportions of 3 to 2, and also of compounds 

 such as. the chlorides of lithium and sodium 

 (regarded in like manner), with certain propor- 

 tions of water; and to urge these as illustrations 

 of the principle assumed by him, that inter- 

 changeability of saturating function between 

 any elements must depend not only on their 

 being capable of transposition in terms of equiv- 

 alent value, but also on their comparative af- 

 finities for the substance to be saturated. His 

 conclusion is, that " Any two radicals are not 

 equal in saturating power for a third, unless 

 they are equal in equivalency and affinity also ; 

 and in most cases of combination there is a 

 residual saturability, due to affinity, enabling 

 the new compound itself to enter into combi- 

 nation." 



Mechanical Energy of Chemical Action. 

 M. Schroder van der Kolk has a long and highly 

 original communication on this subject in Pog- 

 gendorff^s Annalen, cxxii., 439 (July, 1864), 

 from an abstract of which in the Amer. Jour. 

 of Science, January, 1865, we condense a state- 

 ment of the leading principles advanced in it. 

 The author applies the theory of mechanical 

 energy to chemical processes, especially in con- 

 nection with the relations of chemical affinity 

 to heat. 



Citing the conclusions of Deville (see Dissoci- 

 ation, &c.) that by a sufficiently high temper- 

 ature all chemical compounds may be resolved 

 into their elements, while the separated atoms, 

 upon the lowering again of their temperature, 

 in some cases reunite, and in others do not so, 

 he regards this distinction as being a funda- 

 mental one, marking two classes of bodies ; and 

 the result so reached he further connects with 

 the facts observed by Favre and Silbermann, 

 showing that certain combinations are effected 

 with evolution, and others with absorption, of 

 heat. Eejecting the heat consumed in "ex- 

 ternal work," everybody has at whatever 

 temperature above zero an amount of "me- 

 chanical energy " measured by the quantity of 

 such energy in it at zero, plus the quantity con- 

 sumed in effecting molecular changes that have 

 occurred in it above that point, and the quan- 

 tity appearing in the given rise of temperature. 

 An electric spark determines combination be- 

 tween H and O, and during the change much 

 heat is evolved : previous to combination the 

 elements had each a definite quantity of energy ; 

 hence, disregarding the external work, the 



vapor-product, on cooling to the original tem- 

 perature, will contain a quantity of energy less 

 than that of its components by the amount of 

 heat liberated in the act of combination. Now, 

 in order to decompose again the vapor of water 

 at such temperature, precisely this amount of 

 energy must be resupplied. 



"With compound bodies, then, two cases occur ; 

 each given compound contains either more or 

 less energy than that previously possessed by 

 its components. The first class of cases are 

 those in which, during decomposition, heat is 

 given out ; the second, those in which, in the 

 like change, it is absorbed. Heating, then, a 

 body of t\iQ first class to the point of decompo- 

 sition, heat will during that change be given 

 out ; and the components, on cooling, will not 

 reunite, their sum of energy having been by so 

 much diminished. To secure their recombina- 

 tion, either the body must take up heat from 

 the surrounding medium, or its own tempera- 

 ture must fall ; but although it is possible that 

 strong affinity may effect combination even 

 under such circumstances, yet no instances of 

 the kind appear to be known. The conclusions 

 just stated in regard to the first of the two 

 classes of bodies, are confirmed by the facts 

 observed by Favre and Silbermann in respect 

 to nitrous oxide, peroxide of hydrogen, chlorous 

 and chloric acids, and hold true also of the 

 chloride, iodide, and sulphide of nitrogen, all 

 of which bodies, on decomposition by heating 

 or otherwise, evolve heat, and do not recom- 

 bine on cooling. The law holds also in case of 

 certain changes of physical condition in dimor- 

 phous or polymorphous bodies, as when heated 

 arragonite passes into calc-spar, etc. The con- 

 verse of the law intimated the case, namely, 

 of the second of the two classes of bodies is 

 illustrated by carbonate of lime, which, decom- 

 posed by heat, absorbs heat: the compound 

 has less energy than its components taken to- 

 gether have, and as a fact the base and acid re- 

 unite on cooling; also in case of slacking 

 quicklime: the compound CaO + HO evolves 

 heat in forming, and consequently this hydrate, 

 decomposed by heat, forms again upon cooling. 



But here an apparent exception presents 

 itself. In the formation of carbonic acid and 

 of water heat is evolved; decomposition can 

 only take place by absorption of heat, at still 

 higher temperatures; and on cooling, recom- 

 bination ought to occur, but does not so [at 

 least, in all circumstances]. This must be ex- 

 plained on the supposition that, in such cases, 

 the purely chemical affinity does not at the 

 lower temperatures suffice. On this view, two 

 conditions are in general necessary for the 

 formation of a compound : 1, a sufficient chem- 

 ical force, or affinity ; 2, the requisite amount 

 of mechanical energy. Of the two classes of 

 cases above considered, the most frequent are 

 the irreversible, or those in which a body, 

 changing under action of heat and with evolu- 

 tion of heat, does not of itself return to the 

 original state. Of the other or reversible class 



