144 ELEMENTS OF LABOKATORY WORK 



same thermal change in the water. We may therefore say 

 that the compound formed is always at the same temperature 

 under the same conditions. 



If phosphorus be substituted for sulphur in the above 

 experiment, the numerical results will be found to differ. An 

 increase of temperature is observed in the water, and for the 

 same quantity of phosphorus, under the same conditions, the 

 increase is always the same, but it differs in extent from that 

 which would be produced by an equal quantity of sulphur. 

 We may therefore assume, as in the case of sulphur, that the 

 temperature of the compound formed is always the same, under 

 the same conditions, and is always higher than that of its con- 

 stituents in a free state. 



It will be noticed that the compound formed, phosphorus 

 pentoxide, is a solid at ordinary temperatures, although at 

 higher temperatures it is a gas. A comparison of the tempera- 

 tures of the two compounds at the time of formation cannot 

 therefore be made from the final change of temperature 

 produced in the water ; for we have learnt that the change 

 from a gas to a solid is itself accompanied by a thermal 

 change. The comparison can only be made after determining 

 what is the thermal change equivalent to the solidification of a 

 known quantity of phosphorus pentoxide. 



When two solutions act upon another, the thermal 

 change taking place is complex, and its measurement attended 

 with great difficulties. When new substances are formed, a 

 change of temperature is expected from previous observations. 

 We know that the same thermal changes in different kinds of 

 matter are not equivalent ; and we may have here several 

 new substances formed at the same time, under such circum- 

 stances that thermal equilibrium rapidly takes place before the 

 specific temperature of each at its moment of formation can 

 be measured. In some cases there is the further complication 

 of change of state, for one of the new substances formed may 

 be insoluble, and appear in the solid state as a precipitate. 

 These are* some of the difficulties which have to be overcome 

 before correct determinations can be made of the thermal 

 changes which always accompany chemical changes. 



