43 USES OF WATERY VAPOUR IN VEGETATION. 



in which lime acts, when it is employed by the farmer for the purpose 

 of improving his land. [See the subsequent lecture, " On the action of 

 lime when employed as a manure.^^] 



For clay also, water has a considerable affinity, though by no means 

 equal to that which it displays for quicklime. Hence, even in well- 

 drained clay lands, the hottest summer does not entirely rob the clay of 

 its water. It cracks, contracts, and becomes hard, yet still retains 

 water enough to keep its wheat crops green and flourishing, when the 

 herbage on lighter soils is drooping or burned up. 



A similar affinity for water is one source of the advantages which are 

 known to follow from the admixture of a certain amount of vegetable 

 matter with the soil ; though, as in the case of charcoal, its porosity* 

 is probably more influential in retaining moisture near the roots of 

 the plants. f 



3°. The degree of affinity by which the elements of water are held 

 together, exercises a material influence on the growth and production 

 of all vegetable substances. 



If I burn a jet of hydrogen gas in the air, water is formed by the 

 union of the hydrogen with the oxygen of the atmosphere, for which it 

 manifests on many occasions an apparently powerful affinity. But if 

 into a vessel of water I put a piece of iron or zinc and then add sulphuric 

 acid, the water is decomposed' and the hydrogen set free, while the 

 metal combines with the oxygen. 



So in the interior of plants and animals, water undergoes continual 

 tZecomposition and recomposition. In its fluid state, it finds its way 

 and exists in every vessel and in every tissue. And so slight, it would 

 appear, in such situations, is the hold which its elements have upon 

 each other — or so strong their tendency to combine with other substan- 

 ces, that they are ready to separate from each other at every impulse — 

 yielding now oxygen to one, and now hydrogen to another, as the pro- 

 duction of the several compounds which each organ is destined to elab- 

 orate respectively demands. Yet with tlie same readiness do they 

 again re-attach themselves and cling together, wlien new metamorphoses 

 require it. It is in the form of water, indeed, that nature introduces 

 the greater portion of the oxygen and hydrogen which perform so im- 

 portant a part in the numerous and diversified changes which take place 

 in the interior of plants and animals. Few things are really more won- 

 derful in chemical physiology, than the vast variety of transmutations 

 v.'hich are continually going on, through the agency of the elements of 

 water. 



III. In the state of vapour water ministers most materially to the 

 life and growth of plants. It not only rises into the air at 212° Fahr. 

 when it begins to boil, but it disappears or evaporates from open vessels 

 at almost every temperature, with a rapidity proportioned to the previ- 

 ous dryness of the air, and to the velocity and temperature of the at- 

 mospheric currents which pass over it. Even ice a^d snow are grad- 



* Affinity for water causes vegetable matter to combine chemically with it, porosity causes 

 it merely to drink in the water mechanically, and to retain it, U7ichanged, in its pores. 



t For an exposition of the intimate relation of water to the chemical constitution of the 

 solid parts of living vegetables, see a subsequent Lecture, " On the nature and production 

 of the substances of which plants chi^jj sa>Ksist." 



