236 VARIOUS EXPERIMENTS WITH 



however, but always as an ingredient of mixed manures, in 

 which it formed only a small proportion. These experiments, 

 therefore, throw no light upon the special effects of this sub- 

 stance on our different crops and soils. This field of inquiry is 

 consequently still open. 



4. With limes containing magnesia in different proportions. 

 The proportions in each case should be ascertained by analysis,* 

 and they should be tried 



a In comparison with purer limes applied in equal quantities 

 on the same soil, subjected to similar treatment, modes of 

 cropping, &c., that the special effects of the magnesia on 

 grass, root, and corn crops, may as far as possible be ascer- 

 tained. 



b If the effects of pure and magnesian lime be the same in 

 kind, will a smaller quantity of the latter produce an effect equal 

 in amount and equally durable ? 



c In what proportions can a lime which contains a given 

 quantity of magnesia be safely applied to a soil of a given com- 

 position in a given climate ? 



d Is an efficient drainage of the soil any preventive of the 

 injurious effects of magnesian limes ? 



This question is based upon the conjecture thrown out above, 

 that the production of sulphate, or of some other very soluble 

 salts of magnesia, may be one cause of the injurious action of this 

 substance. In well-drained land, the rain that sinks through the 

 soil in wet weather carries downwards, and not unfrequently out 

 of the soil altogether, a portion of the saline matter it contains 

 in excess. In undrained land, this saline matter remains and 

 accumulates, and in dry weather rises towards the surface, and 

 proves a source of injury to the crops that cover it. It appears, 

 therefore, worthy of inquiry how far the effect of magnesian 

 limes is modified by the introduction of what is called thorough 

 drainage. 



I may add, in reference to the experiments with pure caustic 



* I think it likely that burned magnesian limes may not unfrequently con- 

 tain, and yield to water, a sensible quantity of sulphate of magnesia. I have 

 not hitherto ascertained this fact in my laboratory, but the subject is deserving 

 of investigation in connexion with the analyses recommended in the text. 





