88 : THE AMERICAN MONTHLY [May, 
be possible to carry them across the water, but the absence of light for six 
weeks would probably be more than they could stand, to say nothing of other 
unfavorable conditions in the mail bags. However, we propose to try the 
experiment some time, and hope for a favorable result. 
In the course of our photographic work here we have made numerous 
careful observations upon the practical efficiency of pyrogallol and ferrous 
oxalate developers, and a few notes upon this subject may be of interest. 
Ferrous oxalate has always been a favorite with us for its cleanliness, and the 
freedom of the negatives from objectionable color. Many operators contend, 
however, that it does not give sufficient latitude of exposure, and that an un- 
der-exposed plate cannot be made to give the same detail with the oxalate as 
with the pyro developer. We do not propose to enter into this question 
further than to say that, so far as our experience enables us to judge, the one 
developer will do as much as the other. Those who have condemned oxalate 
have usually given formulas for preparing it which seem to us not calculated 
to give the best results. 
Nevertheless, we have come to regard the alkaline developer with greater 
favor than oxalate, for the following reasons :— 
1. It gives softer negatives with short exposures. With ferrous oxalate, 
even when used very weak, a plate slightly under-exposed will give very 
strong and dense high lights before the shadows are well brought up, es- 
pecially if there are strong contrasts in the subject. The only means of con- 
trolling this action is by making the developer weak in iron. Development 
then goes on very slowly, and is a tedious operation. With pyro, on the 
other hand, the details in the shadows may be quickly brought out by adding 
alkali, while keeping the developer weak in pyro prevents undue increase in 
the density of the high lights. 
2. It is a more rapid developer. 
3. It is decidedly more convenient to carry about when one is travelling. 
Pyro stains are quite unnecessary, except when the development has to be 
prolonged with a strong developer. Alum solution, acidified with oxalic or 
other suitable acid, removes any ordinary stain, and with a properly exposed 
plate the resulting color leaves nothing to be desired. Probably the best 
agent to remove stains, after the negative is fixed and thoroughly washed, is _ 
a saturated solution of alum, acidified with oxalic acid, in which a quantity 
of ferrous sulphate is then dissolved. 
We prefer to use the pyro in the dry state, dissolving only what is required 
for each development, at the time it is wanted. The compressed tablets are 
doubtless very good for this purpose, but we use the ordinary dry pyro with 
perfect satisfaction. The common sodium sulphite is not to be compared in 
efficiency with a solution made by saturating a concentrated solution of sodium 
carbonate with sulphurous acid. 
History and biology of Pear-blight. 
By J. C. ARTHUR.* 
1. Hestorical summary. 
Pear-blight, or fire-blight, is a malady which frequently destroys trees in 
fullest apparent vigor in a few hours, turning the leaves suddenly brown, as 
if withered by fire. It happens all through the warm season, most frequently 
in hot and moist weather.t The disease extends from Canada to Louisiana, 
* Abridged from original paper in Proc. Acad. Nat. Sci., Phila., 1886, p. 322. 
+ Cox—Cultivation of fruit-trees, Phila., 1817, p. 174. 
