Dr. Andrews's Report on the Heat of Combination, 511 



experiments as (71, &c.), occasioned by the friction of steam 

 on water held by cohesion to the surface of the discharging 

 orifice ; and that, called the electricity of condensation, pro- 

 duced by the resistance the particles of water offer to the 

 steam, when the steam gives or takes from them motion. 



129. In applying these facts to meteorological phaenomena, 

 we have to add wind to condensation to account for electrical 

 developments ; the friction between air and water producing 

 the electricity, and cloud by its high specific inductive capacity 

 permitting the charge to be maintained. A sudden abundant 

 rain can only be produced by about as sudden an intermixture 

 of masses of the atmosphere, and hence the connexion which 

 always exists between wind and thunder-showers. The 

 structure of hail shows that it has been much exposed to 

 wind*. In addition to those instances I have already men- 

 tioned in which atmospheric electricity may be explained by 

 reference to the experimental facts brought forward, one 

 would expect some electricity to be produced by a fog driving 

 over a wet surface. I would also suggest, that if a mass of 

 ice, water, or vapour should ever enter the atmosphere with 

 great velocity, the steam and friction, together with the cold- 

 ness and rarity of the air in that region, would be very favour- 

 able to the exhibition of electrical meteorous light. 



7 Prospect Place, Ball's Pond Road, 

 May 7, 1850. 



LXIV. Report on the Heat of Combination. 

 By Thomas Andrews, M.Z)., F,R.S., M.R.I.A.f 



I^'HERE are few molecular changes in the condition of matter 

 which are not accompanied by the evolution or absorption of 

 heat. The quantity of heat which is thus set free or absorbed, 

 bears always a definite relation to the amount of the mechanical or 

 chemical action, and its determination in each particular case is a 

 problem of considerable interest as affording a measure of the forces 

 in action. If we consider the great number of phaenomena, mecha- 

 nical, electrical and chemical, among which the production of heat 

 forms the only bond of connexion which has hitherto been clearly 

 ascertained, although there may be strong grounds for suspecting 

 them to be only modified forms of the action of the same force, the 

 importance of investigations of this kind to the future progress of 

 physical science will become at once apparent. 



The object of the present Report is to give a general view of the 

 actual state of knowledge on the subject of thermo-chemistry, under 

 .which we may conveniently include a description of the thermal 



* See Dr. Waller's Observations on Hail, Phil. Mag., vol. xxx. p. 166. 

 t From the Report of British Association for 1849. 



