
ON THE HEAT OF COMBINATION. 67 
press the degrees Centigrade through which one part of water would be raised 
by the heat absorbed in the solution of one part of the salt. 
1. 2. 3. 
590 407 309 
On dissolving 7-99 grms. nitrate of potash in 250 grms. water and repeating 
the operation as before, the successive decrements of temperature observed 
were,— 
oO 
1. 65 C. 5. §=—s B06 C. 
2. 249 6. 1-97 
3 2-34, Te 1°87 
4, 299 8 «1°75 
Combination of Sulphuric Acid with Water—In an elaborate memoir 
on thermo-chemistry, which was published in Poggendorff’s ‘Annalen,’ Hess 
made the first systematic attempt to reduce the quantities of heat disengaged 
in the formation of the hydrates of sulphuric acid to definite laws. His ex- 
periments were made by two distinct methods, which however did not give 
exactly the same results. In the first or indirect method of operating, equi- 
valent quantities of SO,, SO, HO, SO, 2HO, &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 SO, HO added to 
x HO gave a units of heat, and SO, 3HO added to the same x HO gave 6 units, 
a—b was supposed to represent the number of units which would be obtained 
on combining SO, HO and 2HO. In the second, or direct method, each 
compound was combined with the quantity of water exactly required to con- 
vert it into the succeeding compound, and the heat measured 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 de- 
signate by 2 athe heat disengaged in the combination of SO, HO with HO, 
then, according to Hess, the heat set free in the formation of the other hy- 
drates will be 
S OiysritF EO Mea ses, esd nctbevve 8a 
SO,HO+HO,.......:...,.. Qa 
SO QE Oda ion ihc ichere diotescods a 
SO, 8HO ASM Ow obiiosstibis ab oe 
SO,6HO+#HO............ de 
In an early part of his memoir, Hess gives 38°85 for the value of a, but 
‘this he afterwards changes to 46-94, still maintaining however the accuracy 
of the ratios. It is difficult to see how this can be correct. The only expe- 
‘Timent described by Hess on the combination of the anhydrous acid with 
water gave the number 305, which bears to 46:94, not the ratio of 8: 2, but 
Nearly that of 6:5 to 2. Abria obtained a still lower number for the combi- 
Ration SO, with HO. There can therefore be little doubt, if the experiments 
| may be relied on, that the first ratio is too high, It remains to be seen how 
Tar the others haye been confirmed by subsequent investigations. 
ws FZ 
