IN THE PRESENCE OF AIR AND WATER. 405 



favoring temperature, vegetable matter, as we have already seen, 

 undergoes spontaneous decomposition. In the same circumstances 

 Hme promotes and sensibly hastens this decomposition, — altering the 

 ibrms or stages through which the organic matter must pass — but 

 bringing about more speedily the final conversion into carbonic acid 

 and water. During its natural decay in a moist and open soil, organic 

 matter gives off' a portion of carbonic acid gas, which escapes, and 

 forms certain other acids which remain in the dark mould of the soil 

 itself When quick or slaked hme is added to the land, its first effect 

 is to combine with these acids — to form carbonate, humate, &c., of 

 lime — till the whole of the acid matter existing at the time is taken 

 up. That portion of the lime which remains uncombined, either slowly 

 absorbs carbonic acid from the air or unites with the carbonate already 

 formed, to produce the known compound of hydrate with carbonate 

 of lime, — (that compound, namely, which is produced when quick-hme 

 slakes spontaneously in the air — see p. 368.) — waiting in this state in 

 the soil till some fresh portions of acid matter are formed with which 

 it may combine. But it does not inactively wait ; it persuades and 

 influences the organic matter to combine withjthe oxygen of the air 

 and water with which it is surrounded, for the production of such acid 

 substances — till finally the whole of the lime becomes combine^ either 

 with carbonic or with some other acid of organic origin. 



Nor at this stage are the action and influence of lime observed to 

 cease. On the contrary, this result will, in most soils, be arrived at in 

 the course of one or two years, while the beneficial action of the lime 

 itself may be perceptible for 20 or 30 years. Hence there is much ap- 

 parent ground for the opinion of Lord Kames, '• that lime is as effica- 

 cious in its (so called) effete as in its caustic state." Even the more 

 strongly expressed opinion of the same acute observer, " that lime pro- 

 duces little effect upon vegetables till it becomes effete" — derives much 

 support from experience — since lime is known to have comparatively 

 little effect iipon the productiveness of the land till one or two years 

 after its application ; and this period, as I have said, is in most locali- 

 ties sufficient to deprive even slaked lime of all its caustic properties. 



Of the saline compounds, (sahne compounds or salts are always 

 formed when hme, magnesia, potash, soda, &c., combine with acids,) 

 which caustic lime thus forms, either immediately or ultimately, some, 

 like the carbonate and humate, being very sparingly soluble in water, 

 remain more or less permanently in the soil ; others, like the acetate 

 of lime, being readily soluble, are either washed out by the rains or 

 are sucked up by the roots of the growing plants. In the former case 

 fhey cause the removal of both organic matter and of lime from the 

 land ; in the latter they supply the plant with a portion of organic food, 

 and at the same time with lime — without which, as we have frequent- 

 ly before remarked, plants cannot be maintained in their most healthy 

 condition. 



§ 27. Action of mild {or carbonate of) lime upon the vegetable matter 

 of the soil. 



The main utility of lime, therefore, depends upon its prolonged 

 a/)!er-action upon the vegetable matter of the soil. What is this ac- 

 " '  1 - • ^p benefits to which /, gives rise? 



