1 856.] L>r, Playfair on Agrictdtural Experiments. 289 



WEEKLY EVENING MEETING, 



Friday, May 30. 



Sib Roderick I. Murchison, G.C.S. F.R.S. Vice-President, 

 in the Chair. 



Dr. Lyon Playfair, C.B. F.R.S. 

 On the Chemical Principles involved in Agricultural Experiments, 



Dr. Playfair commenced by pointing out the modern views in 

 regard to the food of plants. This may be divided into Air Food 

 and Earth Food. The air food contains carbonic acid, water, 

 ammonia, and nitric acid. Humus, to which great value was 

 formerly attached by vegetable physiologists, is now known to act 

 by its decay as an earth-provider of these substances. Although 

 the soil and plants have the power of absorbing ammonia directly 

 from the atmosphere, still the largest portion must be supplied to 

 them in solution, either in the form of rain or dew. Our know- 

 ledge on this subject is still imperfect. The average fall of rain 

 on an acre may be taken in this country at 2270 tons. Taking 

 the largest results for the ammonia found in rain water, only 30 lbs. 

 of nitrogen would thus be supplied to crops, while a fair crop of 

 cereals, growing in a few weeks only, contains 50 lbs. of nitrogen ; 

 a crop of turnips contains 100 lbs., and one of mangold and clover, 

 1 50 lbs. No doubt nitric acid furnishes a considerable quantity of 

 the nitrogen brought down by rain, but probably the dew is the 

 more active agent, and this falls in proportion as diligence applied 

 to the cultivation of land increases its radiating surface. 



With regard to the earth food, attention was drawn to the 

 essential ingredients in all plants, and to the characteristic quantities 

 of each in crops of different kinds. This was shown by curves, 

 representing the abstracted ingredients of the soil in the usual 

 crops. 



Within certain limits the air food may be viewed as of a con- 

 stant composition and quantity, the diffusion of air equalising it 

 over all districts. But the earth food is constantly varying, both in 

 quality and quantity. Both air food and earth food being necessary 

 conditions of fertility, the sterility or diminished fertility of a field 

 must depend upon that condition of growth which is variable, and 

 not upon that which is constant. The soil (the variable condition) 



