516 Dr. Andrews's Report on the Heat of Combination. 



following numbers, which express the degrees Centigrade through 

 which one part of water would be raised by the beat absorbed in tlie 

 solution of one part of the salt. .ftoiiin ;)iiJ 'to /?)jnuD'jiJ atiJ 'i^vf* 

 ......V,- ,7;i.. ,!.. -,- ,,A |ii .<j..i, <' w= ^leain iqzo vlao ad'X ,Vj.nioo f»d 



•■)idw ,SG8 •isdmun ttgjkovB;^ lajsv/yiriyf jiiyj: "^ aH^ <ilu« sdl to aoiJen 



sfjf jOji dissolving 7*99 grms. nitrate of potash in 250 grms. water and 

 repeating the operation as before, the successive decrements of tem- 

 perature observed were, — 



1. 2°65 C. 5. 2-06 C. 



2. 2-49 6. 1-97 



3. 2-34- 7. 1-87 



4. 2-22 8. 1-75 



Combination of Sulphuric Acid ivith Water. — In an elaborate 

 memoir on thermo-chemistry, which was published in Poggendorff"s 

 'Annalen,' Hess made the first systematic attempt to reduce the quan- 

 tities of heat disengaged in the formation of the hydrates of sulphuric 

 acid to definite laws. His experiments were made by two distinct 

 methods, which however did not give exactly the same results. In 

 the first or indirect method of operating, equivalent quantities of 

 SO3, SO3 HO, SOj 2H0, &c., were respectively mixed with a large 

 excess of water and the increments of temperature observed in each 

 case. The difference between the increments observed on mixing 

 any two compounds with water, was assumed to correspond to 

 the heat due to the combination of the first compound with the 

 number of equivalents of water necessary to convert it into the 

 second. Thus, if SO3 HO added to x HO gave a units of heat, and 

 SO3 3 HO added to the same x HO gave b units, a— 6 was supposed to 

 represent the number of units which would be obtained on combining 

 SOj HO and 2HO. In the second, or direct method, each com- 

 pound was combined with the quantity of water exactly required to 

 convert it into the succeeding compound, and the heat nieasured 

 by observing the increment of temperature of a determinate quantity 

 of water surrounding the vessel in which the combination took place. 

 These experiments have since been repeated by Graham, Abria, and 

 Fabre and Silbermann, but their results do not generally agree with 

 the statements of Hess. 



The fundamental principle laid down by the latter is, that there 

 exists a simple relation between the numbers which express the 

 quantities of heat set free in the formation of the successive hydrates 

 of sulphuric acid. If we designate by 2 a the heat disengaged in the 

 combination of SO3 HO with HO, then, according to Hess, the heat 

 set free in the formation of the other hydrates will be 



;;ij'i 9lqUi SO3 + HO S^ii ^JGii aaatl 



i /ff -Jiin. SO3HO + HO 2«e9iJiJfmnp 9ril 



uij o SOj 2H0 -f- HO . . .'M^n, i-itj . ioj , U(U abioB oiiJin lo 



SO3 3H0-f 3HO ., fuiia^m \iim-o(nQ 



-ijb eeyfci iioui^SOg 6H0-i-a;H0 .*>^*>»».ti>3-i^ \o tt>«»«j4wwO 



