OXYGEN AND ITS SALINE COMBINATIONS 171 



of aqueous vapour (0'4S) remained constant from the ordinary tempera- 

 ture to tJutf of /'-/tick the combustion of detonating gas takes place (but 



temperature from detonating gas at the same temperature. If the water be as vapour 

 the heat evolved = 58 major calories; if asice = 70'4 major calories. A portion of this 

 heat is due to the fact that 1 vol. of hydrogen and vol. of oxygen give 1 vol. of aqueous 

 vapour that is to say, contraction ensues and this evolves heat. This quantity of heat 

 may be calculated, but it cannot be said how much is expended in the tearing apart of 

 the atoms of oxygen from each other, and therefore, strictly speaking, we do not know 

 the quantity of heat which is evolved in the combination of hydrogen with oxygen ; 

 although the number of units of heat evolved in the combustion of detonating gas is 

 accurately known. 



The construction of the calorimeter and even the method of determination vary 

 considerably in different cases. The greatest number of calorimetric determinations were 

 made by Berthelot and Thomsen. They are given in their works Essai de mecanique 

 cJiiiniquc fonilea sur la thcrmucltimie, by M. Berthelot, 1879 (2 vols.), and thermo- 

 chcmische Untersnchu)it/en, by J. Thomsen, 1886 (4 vols.). The student must refer to 

 works on theoretical and physical chemistry for a description of the elements and methods 

 of thermochemistry, into the details of which it is impossible to enter in this work, all 

 the more so because, as has been shown of late, both the theoretical side of this subject 

 and its practical methods are still in an elementary state of development, and must be 

 subjected to improvement in many aspects before thermochemical study can be of that 

 enormous utility to chemical mechanics which was expected from it at the time of the 

 appearance of the first researches in its province. One of the originators of thermo- 

 chemistry was a member of the St. Petersburg Academy of Sciences, Hess. Since 1870 

 a mass of researches have appeared in this province of chemistry, especially in France 

 and Germany, after the leading works of the French Academician, Berthelot, and the 

 Copenhagen professor, Thomsen. Among Russians, Beketoff, Luginin, Cheltzoff, Chroust- 

 choff, and others are known by their thermo-chemical researches. The present epoch_of 

 thermochemistry, in the absence of a steadfast foundation (and the principle of maximum 

 work cannot be counted as such), must be considered rather as a collective one, wherein 

 the material of facts is amassed, and the first consequences arising from them are noticed. 

 In my opinion three essential circumstances prevent the possibility of extracting any 

 exact consequences, of importance to chemical mechanics, from the amassed and already 

 immense store of thermochemical data : (1) The majority of the determinations are con- 

 ducted in weak aqueous solutions, and, the heat of solution being known, are referred to 

 the substances in solution ; yet there is much (Chap. I.) which forces one to consider that 

 in solution water does not play the simple part of a diluting medium, but of itself 

 acts independently in a chemical sense on the substance dissolved. (2) The other chief 

 portion of thermochemical determinations is conducted by the ignition of substances 

 at high temperatures, and as yet we do not know the specific heat of many substances 

 at these temperatures. (3) Physical and mechanical changes (decrease of volume, diffu- 

 sion, and others) inevitably proceed side by side with chemical changes, and for the pre- 

 sent it is impossible, in a number of cases, to distinguish the thermal effect of the one 

 and the other kind of change. It is evident that the one kind of change (chemical) is essen- 

 tially inseparable and incomprehensible without the other (mechanical and physical) ; and 

 therefore it seems to me that thermochemical data will only acquire their true meaning 

 when the connection between the phenomena of both kinds (on the one hand chemical 

 and atomic, and on the other hand mechanical and molecular or between entire masses) 

 is explained more clearly and fully than is the case at present. As there is no 

 doubt that the simple mechanical contact, or the action of heat alone, on substances some- 

 times causes an evident and always a latent (incipient) chemical change that is, a 

 different distribution or movement of the atoms in the molecules it follows that purely 

 chemical phenomena are inseparable from physical and mechanical phenomena. This is 

 because the atomic relations forming the essence of the chemical relations of a substance 

 are not observable, and at present are incomprehensible, without the molecular relations 



