The Potato Disease and its Cause 



197 



After a thunderstorm, the disease may make 

 its appearance, but the electricity or Hght- 

 ning from the storm clouds do not cause it, 

 but electricity produced from the formation of 

 watery particles in the shape of dew-drops. 

 The atmosphere abounds in electricity, but 

 with the production of lightning in storms 

 we do not propose to deal, but to confine 

 our observations to the minute electric fluids 

 (if electricity can be indeed termed a fluid, 

 but for the purpose of our remarks we will 

 call it such), produced by the vapours which 

 are condensed into dew-drops. From ex- 

 periments made by M. Le Monnier, at St 

 Germain en Laye, and communicated to the 

 Academy of Sciences at Paris, the amount of 

 electricity in the atmosphere was estimated. 

 Le Monnier caused a pole to be fixed verti- 

 cally into the ground, which, at the height 

 of 32 French feet, bore an insulated tin point, 

 from which the metallic wire conveyed the 

 electricity to an electroscope, or any other 

 convenient apparatus for its examination. 

 The result was that some electricity might 

 always be detected in the atmosphere ; that 

 in dry weather it was scarcely perceptible at 

 sunrise, but increased gradually until three 

 or four o'clock in the afternoon ; it then 

 diminished till the evening fall of dew, at 

 which time it increased; and by a subsequent 

 diminution it became almost insensible at 

 midnight. The electrical state of the atmo- 

 sphere has been practically investigated by 

 others with equal or similar results. 



From the foregoing it appears that the 

 change of the moisture of the earth, by the 

 heat of the sun during the day, into vapour, 

 produces electricity, and that also the change 

 from vapour into dew causes also an increase 

 of electricity. This change of vapour into 

 visible watery particles in dew-drops is more 

 perceptible on vegetation than, for instance, 

 on gravel walks, &c. This is because vege- 

 tation, such as grass, &:c., being pointed, offers 

 greater facilities for the radiation of the sun's 

 heat, and thereby offers a cool surface to the 

 atmosphere before rounded and smooth ob- 

 jects do, and consequently retain on them a 

 greater amount of watery particles ] and also 

 a greater quantity of electricity is pre- 



sent. Electricity seeks the earth, and the 

 sap of plants frequently forms for it 

 a conductor. The rough, large and 

 pointed leaves of potato-plants are such 

 as to present to the atmosphere a cool 

 surface much sooner than other objects, 

 and consequently they receive a larger 

 quantity of dew. As a sequent in the 

 formation of this dew, especially in hot 

 Aveather after very much rain, in a moist 

 soil, proportionately a larger quantity of 

 electricity is formed in close proximity 

 to the leaves on which the dev/ is deposited, 

 which seeks the best conductor for it into the 

 earth. I'he sap of the soft haulms of the 

 plants aftbrd this, and the electricity is 

 conducted to the tubers. For the same 

 reason, the part of the tubers which contain 

 the most succulent matter and are lowermost 

 in the soil, receive the strength of the current 

 and are destroyed. 



In localities with a southern aspect, 

 especially after rain in summer, owing to a 

 higher temperature in comparison with open 

 districts and with northern or eastern aspects, 

 there is a greater amount of vapour, and also 

 a heavier deposit of dew, so that in such 

 localities the potato disease is known to 

 make its first appearance in the vicinity of 

 the I sea coast, because of saline properties 

 there is always more moisture to be eva- 

 porated, which adds to the formation of dew. 

 Small, smooth, and round-shaped leaves,, 

 and potato plants which do not spread 

 luxuriantly on the surface of the ground sO' 

 as to prevent a free circulation of air, are less, 

 susceptible to disease than those varieties, 

 which, so to speak, grow apace with large and 

 rich foliage. The disease makes its ap- 

 pearance when the greatest amount of 

 moisture is on the ground, and at a time 

 when the foliage is so thick as to prevent 

 a current of air from passing underneath 

 them and dispersing the vapours produced 

 by the sun's rays in the hottest part of a 

 summer's day. These vapours, by being 

 concentrated in the evening, add to the in- 

 crease of the dew deposit, increase the pro- 

 duction of electricity, and thereby generates. 

 the destructive malady. 



