324 
Nature and Cause of the Potato Disease. 
other products, and sufficiently accounts for the alkaline state of a 
diseased tuber. This is the case with the stem and the foliage 
also, which are acid in their action on litmus paper while in a. 
healthy state, but alkaline when putrescent. Pure starch and 
gum contain no nitrogen, they are therefore more stable products, 
and not so easily acted on as azotised bodies; hence their integrity 
in the midst of the decomposition of the other bodies. The 
colouring matter produced by the disease is insoluble in water, 
ether, alcohol, acids, and alkalies. Sulphuric and hydrochloric 
acids dilute, render the colour somewhat lighter, but do not dis- 
solve it. This colour is produced by the presence of oxygen, but 
by no other agent that I have tried. Nitrogen, hydrogen, and 
carbonic acid produce no change of colour, but atmospheric air 
does, and the atmosphere is composed of nitrogen, oxygen, and 
carbonic acid. Now, as oxygen produces the colour, while nitro- 
gen and carbonic acid do not, its appearance must be owing to 
oxidation ; and as this colour is found in the internal parts of the 
tuber, where no direct contact with the atmosphere exists, it seems 
probable that its production in such parts is attributable to the 
liberation of oxygen during the first stages of putrescence, as we 
can only account for its production by the presence of oxygen. 
Healthy tubers, however, contain air, as'may be seen by placing 
them in water previously divested of it under the reservoir of an 
air-pump, and exhausting the receiver. But the air thus confined 
will not account for the colour, as it only appears with the disease. 
All the phenomena of colour observable in a diseased tuber may 
be produced by exposing thin slices, or what is better, grated 
portions of potato, to the action of the atmosphere. Should the 
potato thus treated have lost much of its water by evaporation an 
addition must be made to it, or the action will be slow and uncer- 
tain. The first change that we observe is that of the colour 
becoming yellow, then light brown, reddish brown, deep brown or 
umber, and lastly, black. At the first, or yellow stage, an aroma 
is perceived, similar to that of olive oil ; this increases in inten- 
sity until after the black stage, when putresence sets in, and we 
then lose the olive oil aroma and perceive that of caseic acid. 
The smell of caseic acid is perceivable until a foetid odour arises, 
which is the last stage that presents itself until putrescence has run 
its course, when all smell ceases. 
All the diseased tubers that I have analyzed contained more 
water than the sound ones, generally from 3 to 4 per cent. 
According to Dr. Playfair, the tubers of former seasons con- 
tained from 62 to G8 j)er cent, of water ; the first being the mini- 
mum, and the latter the maximum amount generally found. In tiie 
examinations that I have made 1 have found 78 per cent, in the 
sound tubers of last season, the whole of which were free from 
