Nature and Cause of the Potato Disease, 
329 
5thly. That it was a putrefactive disease, caused by excessive 
moisture, absence of the solar rays, want of exhalation from the 
plant and evaporation from the earth. 
The three first observations approach more to the character of 
facts, as they are founded on the recorded testimony of the great 
mass of observers, and are generally admitted to be occurrences 
that happened during the season ; I shall therefore apply myself 
more particularly to the two last, as it is in these we must look 
for the cause. 
The early crops suffered less than the late ones, as a general 
rule ; and the earlier planted of these less than those of the later 
planted. The plants generally throve until about the middle of 
June, when cold and wet set in ; this state of things continued 
until about August, when the foliage of the plant appeared 
injured, and it was after that pretty generally observed that the 
crops were unhealthy. At the time that the most unfavourable 
weather appeared the late plants were in full work, the tubers 
being then forming. Now, we have seen that a plant cannot 
elaborate its products without the direct action of the sun's rays ; 
and we have further seen that when the vital principle ceases its 
action, even temporarily, that a chemical one commences ; and 
that this commences by an oxidation of the carbon and a libera- 
tion of carbonic acid, and, if this action continue, ammonia is 
formed. 
What is the effect of this ? If ammonia be formed in a plant 
like the potato, which in its normal state is acid, we have not 
only a subversion of the condition of the juices, but we have also 
an agent that dissolves the vessels of the plant, and destroys its 
organism. Here there is an agent equal to effect all that we have 
seen in the disease of the last season. But although we have thus 
briefly arrived at this change, it is a work of time in the plant, 
and necessarily involves many phases. 
1st. We have absence of sun — cold, wet, and want of exha- 
lation. 
2nd. Formation and disengagement of carbonic acid. 
3rd. Generation of ammonia. 
4th. Putrescence. 
During the time the plant was thus acted upon under the con- 
ditions of last season, many appearances presented themselves ; 
for it was not the work of a moment, but a succession of minute 
actions, that gradually undermined the powers of the plant and 
destroyed its vitality. 
When the sun withdrew its vivifying influence the plant was 
in the midst of water, and overpowered by it. Its exhalation was 
impeded, if not totally stopped, and it had an unusual degree of 
