HOW SULPHATE OF MAGNESIA ACTS. 135 



Among root crops, however, there are differences. The 

 potato contains more magnesia than the turnip, while the man- 

 gel-wurtzel, also, contains in its leaf more magnesia, but is 

 not, either in leaf or bulb, so rich in lime as the turnip and car- 

 rot are. 



2. Theory of the action of sulphate of magnesia. If this salt 

 exercise any special action upon vegetation, it may arise from 

 either of three causes. 



a From its extreme solubility, which enables it more readily 

 to enter into plants, and thus to supply them with either of its 

 constituents sulphuric acid or magnesia. 



b From its supplying magnesia to those plants which specially 

 require it. In this case it ought to be grateful to our wheat and 

 other corn plants, and, among our roots, to the potato more 

 than to the turnip and carrot. On the bean, the pea, the turnip, 

 and the carrot, the comparative action of gypsum ought to be 

 greater than that of sulphate of magnesia ; while, to the man- 

 gel-wurtzel and the beet, the two salts ought to be equally, 

 though neither of them largely, propitious. 



c From its peculiar properties as a chemical compound. Like 

 other saline substances, it may act as a compound body in a 

 way which is not to be inferred from what we know of its con- 

 stituents, a mode of action which I allude to as only conjec- 

 tural, yet deserving of rigorous experimental investigation. 



I may here add, however, a remark connected with this sub- 

 ject which is suggested by these salts of magnesia, that sub- 

 stances certainly do appear to perform functions in reference to 

 organic life which are entirely distinct from that of contributing 

 the materials necessary to build up their substance. Thus mag- 

 nesia exists largely in the mineral substance of the seeds on 

 which man lives, and yet it is lime and not magnesia that enters 

 into the composition of his bones, and which is found most 

 abundantly in the other solid parts of his body, and even in 

 his blood. We have every reason to believe that the mag- 

 nesia of the grain does perform an important function in the 

 animal body, and, in fact, is caused to collect in the grain with 

 a view to that function. But this function is not to add to, 

 or to remain in conspicuous proportion in any part of the body; 



