Dr. Andrews's Report on the Heat of Combination. 51 S 



served to hold between the quantities of heat evolved in the com- 

 plete hydration of the first set of salts, or in the combination of the 

 second set with the first atom of water. Neither does it apply totlie^ 

 other sulphates of the magnesian series. - i.ij, ? 



None of the experiments hitherto published furnish all the requi- 

 site data for calculating with precision the absolute quantities of 

 heat set free or absorbed in these cases of chemical action. The 

 weights of the water and of the salt are given, and sometimes the 

 weight and form of the vessel, and the material of which it is com- 

 posed ; but these data are not sufficient to enable us to deduce the 

 true numbers from tlie observed increments or decrements of tem- 

 perature. Knowing the weight and composition of the containing 

 vessel, we may, it is true, calculate its thermal value in water. But 

 other corrections, such as those for the heating and cooling influ- 

 ence of the surrounding air, can only be ascertained by special 

 experiments performed under similar conditions to the original ob- 

 servations. Neither have any experiments of sufficient accuracy 

 been made to determine the specific heats of the solutions formed. 



To complete an investigation which would furnish all these ele- 

 ments, would be a work of very great labour, and will probably 

 scarcely be undertaken till our instruments and means of observation 

 are greatly improved. As a first step to such an inquiry, I may here 

 describe a few preliminary experiments on the specific heats of some 

 saline solutions, and on the quantities of heat absorbed in the solu- 

 tion of successive portions of the same salt, 



To obtain results approaching to accuracy in experiments on the 

 specific heats of saline solutions is extremely difficult, as the errors 

 of experiment are often of nearly the same order of magnitude as 

 the whole differences to be observed. The corrections for the cooling 

 and heating action of the air and for the effects of radiation, cannot 

 be estimated with any certainty by the application of general for- 

 mulas founded on experiments made at a different time* ; and the 

 most careful examination of the calibre of the thermometer tube 

 will fail to render different parts of the scale accurately comparable 

 with one another to a five-hundredth part. The general method 

 pursued in the determination of the following specific heats was the 

 same which I described some years agof ; but to avoid the uncer- 

 tainties just referred to, alternate experiments were made with pure 

 water and with the solution, under conditions as nearly as possible 

 identical, and these were repeated till accurate means were ob- 

 tained. By this mode of operating, a very great degree of preci- 

 sion may be given to experiments of this kind. 



The only salts whose solutions have yet been examined are the 

 nitrate of potash, the nitrate of soda and the chloride of sodium. 



* If the vessel be uncovered, changes in the hygrometric state of the atmosphere 

 produce a very marked influence on the rate of cooling, when the excess of tempe- 

 rature above the air amounts only to a few degrees ; and even in a close apartment 

 the increased agitation of the air on a windy day sensibly increases the rate of 

 cooling. •' 



+ Philosophical Transactions for 1844, p. 34. iioji^ii/ ; i aj^jiqinoi 



