376 
On the Potato Disease. 
during which time it was incapable of perfectly evaporating lis 
redundant juices for want of sunshine ; they accumulated in con- 
sequence, and became vitiated by stagnation in that crude state ; 
and when fine hot weather ensued afterwards, the sudden rush of 
sap was too great for its debilitated tissues, and the diseased fluid 
showed itself at the surface in various places, on the leaves and 
stalks, sometimes in spots not larger than a pin's head, at others 
in blotches, which rapidly increased if the stimulus of heat was 
kept up. 
The virus of these spots, being of a phagedenic or ulcerous 
nature, eventually descended into the potatoes. 
The effect upon these has been already described at the com- 
mencement of this essay, and has been also so well shown by Dr. 
Lyon Playfair in his comprehensive lectures, that it must be quite 
unnecessary to repeat it, particularly as he has illustrated the 
subject by such accurate diagrams. 
As lo the chemical action of the disease I am not competent to 
give an opinion. Dr. L. Playfair has stated that he considers it 
to be owing to oxidation of the debilitated tissues. But Mr. E. 
Solly attributes it to putrefaction of azotized matter in the stems, 
but also implies that it might arise from oxidation, if the tissues 
were debilitated, which he, however, doubts. But for my own 
part, I cannot conceive it possible for plants to be rendered in- 
capable, during a long period of the growing season, of performing 
their most important functions toithout their being enfeebled, and 
thus, if this point were conceded, as I believe it generally is, 
very little difference would exist on this head between two of the 
most eminent agricultural chemists in this country. 
Much has been said about the disease being contagious. 
Strictly speaking, it is not so, for I had three roots of Shaws 
taken up on the 29th of August, containing twelve sound and 
nine unsound tubers, which I placed on the bare ground, taking 
care to make the diseased parts touch the sound ones. I then 
covered them over with their own haulm and an old mat. I 
examined them frequently, and lastly, on the 21st of March, when 
the twelve sound ones were still without blemish, eight of the 
others were diseased all over, and one had only the upper eyes 
sound. Ill February last 1 also had twenty bushels of perfectly 
sound potatoes, picked out of a heap of bad ones with which they 
had been thrown when they were dug up. I have since had 
these cut, and, finding them perfectly sound, used them for set^;. 
It was supposed, from an experiment made by Mr. VV. Crum 
of Glasgow, that all potatoes last year contained the principles 
of disease, in consequence of the pulp when grated turning first 
ferruginous and afterwards black. But I have found that, if the 
potato is scraped with an ivory knife instead of an iron grater. 
