New Yorx AgricutturaL Exprrment Station. 667 
of the ferrocyanide test admitted that he stopped adding lime 
while the test still showed a slight change in color. He did not » 
realize the importance of adding lime until the ferrocyanide would 
not change color when dropped into the mixture. This incident 
shows how important it is in recommending the ferrocyanide test 
to impress on the minds of those who are to use it the necessity 
of carefully following instructions as to the proper method of its 
use, and while an excess of lime is not injurious a mixture con- 
taining too little lime will surely work injury. But because some 
people have failed to use this test rightly is certainly no argu- 
ment against its reliability when rightly used. The use of the 
test offers a most convenient method of preparing the Bordeaux 
mixture, and it is also reliable, as many experiments have shown. 
As stated by Dr. Van Slyke, before the Western New York 
Horticultural Society, January 23, 1895,” by means of this test 
one part of copper may be detected in about 400,000 parts of 
_ solution, certainly a very delicate test. It would be well as 
before stated, whenever employing this test in the preparation 
of the Bordeaux mixture, to add more lime after the test shows 
no further change of color and thus insure an excess of lime. 
Iodine and Starch Paste Test. 
In the address above referred to Dr. Van Slyke described a 
test even more delicate than the ferrocyanide test which may be 
conveniently used in preparing the Bordeaux mixture. While 
that detects one part of copper in 400,000 parts of solution, 
this one shows one part of copper in five hundred thousand parts 
of solution. It is used as follows :— Some of the solution to be 
tested is taken in a white china teacup and to this is added a few 
drops of ordinary starch paste and then a few drops of a solution 
of potassium iodide. If any copper sulphate is in the solution, 
it will act upon the potassium iodide, setting free the iodine, 
which will color the starch blue. The color is more readily seen 
against a white background, such as a china cup offers. The 
starch paste is such as is used in starching clothes. 
The solution of potassium iodide may be prepared by dissolv- 
ing one ounce of the compound in a quarter of a pint of 
water. 
22 Proc, West. N. Y. Hort. Soe’y, 1895, pp. 108-109. 
