The preparation of ferrocyanide of potassium for this test may be explained. As bought at the drug store, it is a yellow crystal and is easily soluble in water. Ten cents worth will do for a season's spraying of an average orchard. It should be a full saturation; that is, use only enough water to dissolve all the crystals. The cork should be notched or a quill inserted so that the contents will come out in drops. A drop will give as reliable a test as a spoonful. The bottle should be marked "Poison." Dip out a little of the Bordeaux mixture in a cup or saucer and drop the ferrocyanide on it. So long as the drops turn yellow or brown on striking the mixture, the mixture has not received enough lime.
"Process" Lime for Bordeaux Mixture.
The so-called "new process," or prepared limes, now offered on the market, are of two classes. One consists of the quick-lime that has been ground to a powder. The other is the dry water-slaked lime made by using only enough water to slake the quick-lime, but not enough to leave it wet. Practically all of the process lime on the market is the ground quick-lime.
When the hard "stone" lime becomes air-slaked it is evident to the eye from the change to a loose powdery mass. Should one of these prepared limes be to any considerable degree air-slaked, its appearance would be no indication of its real condition.
A simple test for the presence of much carbonate of lime in these prepared limes, can be easily performed, a small amount of lime — 1/4 teaspoonful — dropped on a little hot vinegar, will effervesce or "sizzle" if it contain the carbonate of lime, acting about the same as soda.
A sample of a new process lime analyzed at this Station showed 30 per cent, magnesia. This came from burning a dolomitic limestone, that is, one containing carbonate of magnesia with the carbonate of lime. The magnesia does not slake with water like the lime and hence is useless in the Bordeaux mixture. There is no easy way outside a chemical laboratory of telling the presence of magnesia.
As a general rule more "process" lime is required to neutralize the copper sulfate than good stone lime. It is always well to make Bordeaux mixture by using the ferrocyanide of potassium test — Cornell University.