170 



PRINCIPLES OF CHEMISTRY 



The combination of hydrogen with oxygen is accompanied by the 

 evolution of a very considerable amount of heat ; according to 

 the determinations of Favre and Silbermann* 1 1 part by weight of 

 hydrogen in forming water evolves 34462 units of heat. Many of the 

 most recent determinations are very near this figure, so that it may be 

 taken that in the formation of 18 parts of water (H 2 O) there are 

 evolved 69 major calories, or 69000 units of heat. 38 If the specific heat 



at 450-560 J , when detonating gas explodes, the density of aqueous vapour not only 

 does not vary (and it hardly varies at higher temperatures, probably because the amount 

 of the products of dissociation is small), but there are not, as far as is yet known, any 

 traces of dissociation ; (2) that under the influence of contact the temperature at which 

 combination takes place falls even to the ordinary temperature, when water and similar 

 compounds naturally are not dissociated and, judging from the data communicated by 

 D. P. Konovaloff (Introduction, Note 39) and others, it is impossible to escape the phe- 

 nomena of contact ; all vessels, whether of metal or glass, show the same influence as 

 spongy platinum although to a much less degree. The phenomena of contact, judging 

 from the mass of the data referring to it, must be especially sensitive in reactions which 

 are powerfully exothermal, and the explosion of detonating gas is of this kind. 



57 The amount of heat evolved in the combustion of a known weight (for instance, 1 

 gram) of a given substance is determined by the rise in temperature of water, to which 

 the whole of the heat evolved in the combustion is transmitted. A calorimeter, for 

 example, that shown in fig. 33, is employed for this purpose. It consists of a thin (in 

 order that it may absorb less heat), polished (that it should transmit a minimum of heat) 

 metallic vessel, surrounded by down (c), or some other bad conductor of heat, and an outer 

 metallic vessel. This is necessary in order that the least possible amount of heat should 

 be lost from the vessels ; nevertheless, there is always a certain loss, whose magnitude 



is determined by preliminary experiment (by taking 

 warm water, and determining its fall in temperature 

 after a definite period of time) as a correction for the 

 results of observations. The water to which the heat 

 of the burning substance is transmitted is poured 

 into the vessel. The stirrer g allows of all the layers 

 of water being brought to an equal temperature, ;m<l 

 the thermometer serves for the determination of the 

 temperature of the water. The heat evolved p;is>c>, 

 naturally, not to the water only, but to all the parts (A 

 the apparatus. The quantity of water corresponding 

 with the whole amount of those objects (the vessels, 

 tubes, &c.) to which the heat is transmitted is pre- 

 viously determined, and in this manner another most 

 important correction is made in the calorimetric deter- 

 minations. The combustion itself is carried on in the 

 vessel a. The ignited substance is introduced through 

 the tube at the top, which closes tightly. In fig. 3& 

 the apparatus is arranged for the combustion of a gas, 

 introduced by a tube. The oxygen required for the 

 combustion is led into a by the tube <?, and the pr<>- 



PIG. 33. Favre and Silbermann's calo- ducts of combustion either remain in the vessel a (if 



"volve'd ifcombSln.^ "" ^ li( l uid or solid), or escape by the tube/ into an n 1M Kua- 



tus in which their quantity and properties can easily 



be determined. Thus the heat evolved in combustion passes to the walls of the vessel a, 



and to the gases which are formed in it, and these transmit it to the water of the 



calorimeter. 



58 This quantity of heat corresponds with the formation of liquid water at the ordinary 



