234 EXPERIMENTS WITH MAGNESIA SUGGESTED, 



It may nevertheless be, that natural differences in the lime 

 do modify this peculiar virtue, and that the different results in 

 regard to the straw, which are observed in some soils and dis- 

 tricts, may be connected with these natural differences. 



When shell-sand, the purest varieties of which always contain 

 a certain proportion of siliceous sand, are burnt in a kiln, they 

 yield a lime in which a considerable proportion of silicate is al- 

 ways present. This burnt shell-sand has lately been recom- 

 mended very highly as an application to the land, and the value 

 of the silicates it contains has been much extolled. Experiments 

 which shall clearly make out the precise effect of the silicates in 

 our different varieties of lime, are, therefore, much to be 

 desired. They will prove both theoretically and economically 

 useful.* 



7. Suggestions for experiments with magnesia, and with limes 

 which contain it in considerable proportions. 



All limes, as I have said, contain magnesia, but those called 

 dolomitic or magnesian often contain it in large proportion. In 

 the magnesian limestones of the county of Durham, the carbonate 

 of magnesia varies from 2 to 45 per cent of their whole weight. 



In this district, the value of a lime for agricultural purposes is 

 determined very much by the proportion of magnesia it con- 

 tains. It is very desirable, therefore, to ascertain the precise 

 nature of the effect produced by magnesia upon different soils 

 and crops. Is this effect also produced equally by caustic or 

 burned magnesia, and by carbonate of magnesia ? Is the car- 

 bonate, as some have stated, harmless, or is it merely more slow 

 in its action ? On what property of magnesia does its injurious 

 action depend? Does the extent or rapidity of its injurious ac- 

 tion depend in any degree on the nature of the soil to which 

 it is applied? 



In my published Lectures I have thrown out the conjecture, 

 that the tendency to retain the caustic form for a longer period 

 than the lime with which it is mixed, may be a cause of the 

 injurious action of magnesia. It has since occurred to me, from 



* See my Contribution to. Scientific Agriculture, pp. 59 and 125 ; and Use of 

 Jhinie in Agriculture, p. 23, 



