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2; To find some medium outside the animal body, in 
which this organism will live and multiply. 
3. To cultivate the organism in this medium for a 
sufficient number of generations to insure the complete 
elimination of other organisms that may have been intro- 
duced into the first cultivation; in other words, to secure a 
pure cultivation of the organism. 
4. To inoculate a healthy individual from the pure 
culture of the organism, and produce the original disease. 
These steps carefully followed, afford a means of proof 
that, it seems to me must convince the most skeptical. 
This method of proof is just as applicable to plan ^ diseases 
as to animal, and in the case of pear blight it remained for 
Professor Arthur, then of the New York experiment station 
at Geneva, to apply it. This he did during the seasons of 
1884 and 1885. 
WORK OF PROFESSOR ARTHUR. 
Professor Arthur used as a culture medium a tea made 
by steeping corn meal in water and then filtering until a 
clear infusion was obtained. In this medium he cultivated 
the organism for a number of generations. Trees inocu- 
lated from his last culture, which contained Micrococcus 
amylovorus, and no other organism, developed the disease. 
Here was good proof that this specific organism caused pear 
blight; but there was one question that might be raised. 
Might not the liquid in which the organism lived be the ex- 
citing cause, instead of the organism ? To prove this point 
a culture containing the organism was filtered through por- 
celain. The clear liquid, which upon examination by the 
microscope was shown to be free from germs, failed in 
every case to communicate the disease, but the residue of 
germs, left after filtering, when used to inoculate healthy 
trees, readily produced the disease. Thus by the method of 
experiment has every doubtful point been covered, and the 
fact established beyond controversy that this particular 
organism. Micrococcus amylovorus, is the true cause of pear 
blight, or apple blight. 
This demonstration did not at once meet with universal 
acceptance. Various objections were raised to it. There 
were many men who refused to accept as the exciting cause 
..something they could not readily see, something which 
could not readily be made evident to the senses. The ob- 
servation and study of these low organisms, and of the 
tissue in which they live must be carried on under high 
powers of the microscope; they must be magnified at least 
