277 
cates, to the class of alkaloids called ptomaines. To decide 
the matter of their existence tests were made in April last 
as follows : 
A culture of blight bacteria in about 200 cc. of infusion 
of potato, giving a vigorous growth with zodgloea, was filter- 
ed and evaporated nearly to dryness, redissolved:in alcohol, 
and then tested with potassic ferric-cyanide, phospho- 
molybdic acid, and potassio-mercuric iodide, all important 
reagents in detecting ptomaines.! These tests gave no in- 
dications of the presence of such compounds. To be doubly 
sure, the liquid was made strongly alkaline with potassic 
hydrate, and the tests repeated ; the results were the same. 
About the same amount of material was prepared by cul- 
tivating the bacteria in a preparation of potato and water, 
cooked over a warm bath, and sterilized. This was filtered, 
the residue treated with 100 cc. of distilled water, slightly 
acidulated with hydrochloric acid, heated to 150° F., and 
filtered. The two filtrates were united, evaporated, and 
digested in the cold with alcohol made alkaline with sodic 
hydrate. This solution was tested with the same reagents 
as before, and also with platinic chloride and concentrated 
sulphuric acid, with similar negative results. 
Another trial, using a cultivation in a boiled potato 
having only the usual amount’ of moisture, was made by 
treating with alcohol acidulated with tartaric acid, heating 
to 150° F’., the solution filtered and evaporated nearly to 
dryness, and redissolved in distilled water. After an hour 
it was tested as before, and with uniformly negative results. ? 
These tests, carried out under the direction of Mr. E. F. 
Ladd, assistant chemist of the Station, and a special student 
of ptomaines, make it very certain that pear blight bacteria 
produce no poisonous alkaloids during their growth in arti- 
ficial cultures. No other chemical products of bacterial 
growth are known to actively promote disease, and the 
proof is therefore sufficiently complete, that, whatever may 
be true regarding the growth of the bacteria in the tree, 
they at least do not behave in artificial cultures in a way to 
vitiate the force of our third statement. 
The fourth statement (d), however, already covered this . 
point by implication, for if the fresh juices from the diseased 
part of the tree would not reproduce the disease when the 
bacteria were removed, it would not be probable that del- 
eterious products would be subsequently formed in the arti- 
ficial cultures that could do so. It 7s therefore proven, by both 
direct and circumstantial evidence, that a specific kind of 
bacteria is the real and only cause of pear blight. 
1See Brieger Ueber Ptomaine, 1885, p. 22 et seq. 
* These experiments have already been published in Proc. Phila, Acad. Sci., 
1886, p. 338. 
