POTATO ROT. 667 



drogen. Therefore, when there are carbonates in the soil, or a 

 due supply of carbonic acid in the air, the potato plant takes 

 the carbon, and sets the carbonic acid free. 



The cause of the decay or rot therefore must be a deficiency 

 of the carbonic acid in the air. Why ? 1. Because of the 

 decrease oi the carbonic acid which has been goi7ig on for ages., 

 in consequence of the growth of the whole vegetable kingdom. 

 2. Because the disease has been prevented just so far as the 

 root and plant have been supplied with carbon — by charcoal, 

 carbonate of lime, or any substance rich in carbon. 



The carbonates, or preventives, to be eifectual, must be so 

 applied that the potato can draw from it. 



The deficiency of the carbon (the cause of the disease) is 

 both in the soil and atmosphere. But a newly cleared and 

 burnt piece of ground, it is said, will produce sound potatoes. 

 Now if vegetation absorbs carbonic acid, why this result, since 

 there was so recently a large growth of wood on the land now 

 cleared ? The reason is obvious. There is a supply of carbon 

 from the charcoal and ashes left on the ground after burning, 

 as well as in the soil, which has not been overworked and ex- 

 hausted, but has rather been supplied from the falling and de- 

 cay of the carboniferous growth of ages before. 



During the earlier periods of the earth's history, the atmo- 

 sphere was much more highly charged with carbonic acid, than 

 at the present time. Of this there is abundant proof; and to 

 this redundancy of carbonic acid may be attributed the luxuri- 

 ant growth of vegetation peculiar to those periods ; which 

 growing up and falling down, have produced those vast beds 

 of coal, or disintegrated vegetable matter, which remain to the 

 present time, and fully indicate a much larger growth of for- 

 ests, than are at present known. 



Coal is composed of vegetable matter, transmuted by petre- 

 faction of a peculiar kind, beneath the surface of the water, 

 and in the absence of air. Many species of plants which grew 

 luxuriantly then are now extinct. They passed away with 

 the decrease of carbonic acid gas that supported them. They 

 lived till they fulfilled the important work assigned them, viz., 

 clearing the atmosphere of a redundance of carbonic acid — a 



