434 M. Schroder Van der Kolk on the Mechanical 



In the following* considerations two principles are used : — 



I. When mechanical work is performed an equivalent quantity 

 of heat disappears, and conversely ; the same is presupposed with 

 regard to chemical work. 



II. Heat cannot of itself pass from a warmer to a colder body. 

 In due connexion with this is the principle that mechanical or 



chemical work can of itself be spontaneously transformed into 

 heat ; but heat can never be changed into work without the si- 

 multaneous occurrence of a compensating action — for instance, 

 when a quantity of heat passes from a higher to a lower tem- 

 perature. 



Most important in these determinations is the heat of combi- 

 nation, the determinations of which were mostly made by Favre. 

 He expresses the numbers in thermal units, and refers them to 

 equivalent quantities if the equivalent of hydrogen is taken = 1 

 gramme. The two constituents of a compound unite at a de- 

 finite temperature ; if, then, the compound formed be cooled to 

 the original temperature of the constituents, the heat developed 

 during the operation is the number given by Favre. This tells 

 how much more energy is accumulated in the components in the 

 original condition than in the combination in its final stage. Of 

 course the constituents and the combinations must be in the 

 same state of aggregration, or otherwise the latent heat must be 

 taken into account. If t is the initial and final temperature, t 

 that at which combination takes place, the quantity of heat A is 

 absorbed in raising the body from t to t ; the heat of combination, 

 W, is disengaged at t, as well as the quantity of heat B if the 

 product is cooled down from r to t. FavreVnuinberF = W + B — A 

 gives us the difference in energy ; the real heat of combination 

 is W. The number W gives us the chemical work consumed in 

 the combination. 



The author applies these considerations to chemical actions. 

 Chemical combinations may be divided into two classes, accord- 

 ing as the compounds have less or more energy than the consti- 

 tuents. Water may be taken as a type of the first; heat is libe- 

 rated during combination and absorbed in decomposition. For 

 the second class, of which protoxide of nitrogen is the represen- 

 tative, the phenomena are reversed. Moreover all bodies are 

 decomposed at a sufficiently high temperature. 



The compound gases may be formed as follows : — 



(1) On being passed through a heated tube. Both kinds of 

 the gases as above distinguished may thus be formed. In his 

 considerations the author takes the following division as a basis. 



A. The gases have less energy than their components, and 

 are formed on simple heating. 



