24 On the Heat evolved in Chemical Combination. 



evolved after the results had been corrected for the specific 

 heat of each acid used. 



For this simple method of mixtures M. Hess subsequently 

 substituted the use of a calorimeter, devised by him for the 

 purpose. This instrument consists, 1st, of a box for holding 

 water; inside of this is, 2nd, a cylinder, fitted, as well as its 

 cover, with a broad edge ; these edges are ground and fitted 

 tight to each other by eight screws. The outer surface of 

 the cylinder is fitted with projecting copper wings, which serve 

 to agitate the water in the external box, leaving free, how- 

 ever, a space, which admits of the entrance of the thermometer. 

 The cylinder is supported horizontally on its axis, and is 

 fitted on one side outside of the box with a winch handle, by 

 which it may be made to revolve rapidly. 



The mode of using this instrument is as follows : the cylin- 

 der being taken out is placed steadily in a wooden support, 

 and the cover screwed off. Whichever of the two liquids to 

 be mixed occupies least volume, is to be then introduced into 

 a glass or copper vessel and laid in the cylinder ; the other 

 liquid is to be then poured in, the cover fastened, and the cy- 

 linder laid in its place, surrounded by the water of the cylin- 

 der. On then turning the cylinder rapidly round by the 

 handle, the inside vessel overturns, the liquids mix, and the 

 heat evolved is imparted to the water, outside of which the 

 temperature is accurately determined by a Collardeau ther- 

 mometer. The change of method when one substance is 

 solid can easily be appreciated. 



By means of experiments conducted with this apparatus, it 

 was found, after applying all necessary corrections, that 



Units of Heat. 

 H O + S O s combining with H O evolved 77*17 

 HO + SO3 » 2HO „ 116-7 



2HO + SO3 » HO „ 38-56 



S 3 „ x, H O „ 510-1 



From these results, combined with those already given, the 

 following final results may be deduced for the quantities of 

 heat evolved in the formation of diluted sulphuric acid of de- 

 finite constitution : — 

 Acid. 



S O, + H O 



HO.SO^+HO 

 (2HO + SOJ + HO 

 (3 H O + S 3 ) + 3 H O 



(8HO+SOj + xHO 



Total for S 3 + x H O = 504-96 13 



