16 ON THE USE OF LIME, MARL, AND SHELL-SAND. 



On the Value of Limestones for Agricultural purposes. It has 

 been assumed that a limestone is the more valuable for burning 

 the more lime and the less magnesia it contains. Magnesia is 

 often considered prejudicial to vegetation, and for this reason 

 limestones containing much magnesia are deemed unfit for burn- 

 ing. It is true many magnesian limestones on burning produce 

 poor lime that slakes with difficulty, and does not swell much on 

 slaking, and such lime of course cannot act so beneficially upon 

 vegetation as other varieties, which fall to a fine floury powder 

 and are richer in pure lime. But it does not appear to me logical 

 to infer from the different results which good and bad limes 

 produce upon vegetation that magnesia is injurious to vegetation. 

 Direct experiments which have been made with caustic mag- 

 nesia indeed have shown that there is no foundation for the 

 alleged injurious effects of caustic magnesia, which is supposed 

 by many practical and scientific men to burn up vegetation. It 

 appears also that this erroneous view respecting the action of 

 magnesia upon plants arises from the purely accidental circum- 

 stance that bad limes often contain much magnesia. Space 

 prevents me from discussing in this place at length the erroneous 

 opinion of those who regard magnesia as a deteriorating element in 

 the estimation of the value of an agricultural limestone. I content 

 myself by expressing my conviction that 2 or 3, or even 10 per 

 cent, of magnesia, will not materially affect the value of the lime- 

 stone, if it furnishes a lime which slakes well and falls to a fine 

 voluminous powder rich in lime. It should be borne in mind, 

 however, that slaked lime is used in agriculture for the sake of 

 the lime, and not for the magnesia which it contains. Mag- 

 nesian limestones often contain 20 or even 30 per cent, of car- 

 bonate of magnesia, and do not yield quicklime so rich in pure 

 lime as limestones containing naturally more carbonate of lime. 



There is another mistake which is frequently made, namely, 

 the mistake of connecting the agricultural value of limestones 

 with the greater or smaller amount of shells and other fossil 

 remains which different specimens present to the eye. It is 

 believed that the more shelly or coralline a limestone is, the 

 better the lime which it is likely to produce. This supposition 

 rests on the assumption that corals, shells, and other fossil re- 

 mains in limestones contain much phosphoric acid. Neither 

 the alleged fact, that shelly limestones produce better lime than 

 stones comparatively free from fossil remains, is proved, nor 

 does the statement which has been made in explanation of this 

 assumption rest on analytical evidence. 



Many exceedingly shelly limestones which I have had an 

 opportunity of examining, notwithstanding their shelly texture, 

 are totally unfit for burning ; and some of the worst limes in the 



