The Sysfcm of Husbandry. 33 



separated by the action of heat into its oxide and carbonic acid 

 gas ; next, that the calcined lumps must be distributed in 

 heaps on the field, and that the first rainfall will not only pul- 

 verise them into powder minute enough for assimilation by 

 plant life, but will re-form the oxide into carbonate by acting 

 as the medium for its re-absorption of carbonic acid gas from 

 the air. And since no such process was known to Pliny, we 

 can well imagine that when left in lumps and roughly distri- 

 buted, its fertilising efi'ects would be delayed, and the reaping 

 of the first crop or so would be attended with some difficulty.^ 

 The mechanical effect that much lime has, especially on light 

 soils, in loosening the surface and sometimes throwing out the 

 young corn roots will perhaps explain the author's meaning 

 when he speaks of an excessive dressing as burning up the 

 land. He mentions eight varieties of this earth, some rough 

 and gritty to the touch, others plastic and greasy, and nearly 

 all of dissimilar colouring. Possibly he is mixing up under one 

 common classification all compounds of clay and lime, and even 

 clay and sand, which we call marls and loams, but which were 

 used in his time under the generic term " marga." Then too 

 the fullers' earth of commerce, potters' clay, and shale may 

 account for some of the varieties he mentions ; but his descrip- 

 tion of their appearance and effects as fertilisers is too vague 

 and untrustworthy for us to speak with any degree of cer- 

 tainty. Considered simply as a manure, we know- from the 

 experiments of the laboratory that the proportion of lime 

 existent in any of Pliny's compounds would regulate its value ; 

 but marl, lime, and even clay are often used as top dressings 

 only for the sake of their mechanical effects on the soil. Since 

 however the crops would be thus indirectly benefited, no one 

 but a modern analytical expert could have separated out the 

 chemical from the mechanical fertilisers in Pliny's eight 

 varieties. 



That any of these, when sparsely laid on the land, will 

 fertilise it for eighty, fifty, or thirty years, is an exaggeration 

 which, if even bordering on truth, would upset the compen- 

 satory clauses of many an agreement drawn up under the 



' Plinius, Hist. Nat., lib. xvii. cli. 4. 



D 



