268 Dr. Woods on t/te Heat of Chemical Combination. 



and duration of this '^ rain of bounty '^ are, like as on tlie Abes- 

 sinian plateau, iiTegular and uncertain, we may fairly infer that 

 it has at times no sensible effect on the volume of water in the 

 Nile. Hence we may understand how it happened that in the 

 year 1811 the river, so far from rising in the middle of Januaiy, 

 as it did in 1850, continued falling till the end of that month*. 



In Lower Egypt, precisely at the period of the regular equa- 

 torial rains, namely, " during the months of April, May and 

 June, the waters of the Nile are at their lowest level. Towards 

 the end of June the river at Cairo begins to rise, without the 

 occurrence there of any rainy season, and without the existence 

 of the slightest apparent cause. The increase of the Nile usu- 

 ally continues three months, from the summer solstice to the 

 autumnal equinox, when its waters again begin gradually to 

 fallf/' 



I refrain from discussing here the effect of the flooding of the 

 various head-streams of the Nile on the inundation of that river 

 in Egypt ; merely remarking that its occasional abnormal and 

 momentary increase appears to be solely attributable to the fall 

 of rain in the eastern mountains of Egypt and Nubia : for in- 

 stance, the extraordinary rise of the river observed at Cairo in 

 May 1843 was caused by the rain-waters collected and brought 

 down by Wady Ollaky in about 23° N. lat-J 

 August 19tli, 1851. 



XXXIX. On the Heat of Chemical Combination, 

 By Thomas Woods, M.D. 



'- ' To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, Parsonstown, July 1851. 



IN the course of some investigations respecting the cause of 

 the heat of chemical combination, I found that some facts 

 hitherto unnoticed, or not sufficiently attended to, required to be 

 proved. This I have endeavoured to do ; and will, if you allow 

 me, publish a few papers in the Philosophical Magazine illustra- 

 tive of these facts, which I intend to employ as data in establish- 

 ing a theory I have formed on the subject. 

 ;' The first of these propositions is, that the decomposition of a 

 compound body gives rise to as much cold as the combination of its 

 elements produces heat. 



* Weme, pp. 330, 334. 



t Ehrenberg, in Monatsherichte d. Akad. d. Wissensch. in Berlin (May) 

 1851, p. 334. 



X Joum. Roy. Geogr, Soc. vol. xx. p. 292; and see Bui'ckliardt's Travela 

 in Nubia, p, 10. 



