52 Letters on the Diseases of Plants. 



not to be necessary. Now add the whitewash slowly to the sulphate solution 

 until the latter is neutralised, which can easily be ascertained by testing the 

 mixture with a solution of ferrocyanide of potassium, a yellow crystalline salt 

 to be had of any chemist. Buy sixpenny-worth — it will last a long time. 

 Dissolve the sixpenny -worth of ferrocyanide of potassium in a tumbler of 

 water and place it in a bottle. It will keep ; be careful with it, however, as 

 it is very poisonous if taken internally. The solution, if properly made, will 

 be light straw-coloured. 



To test the Bordeaux mixture so as to find out when suflGcient lime has been 

 added to the 10 gallons of copper sulphate solution, take a drop of the mixture 

 on your iinger and daub it on to a board, or better, a bit of white paper, and 

 add a drop of the ferrocyanide solution with a finger of the otlier hand. If 

 you have not added sufficient whitewash, the ferrocyanide will produce at 

 once a red colour. Keep adding whitewash until the ferrocyanide just fails to 

 produce at once or after a few moments a decided red colour. When that 

 point is reached the copper sulphate is exactly saturated or neutralised by 

 the lime of the whitewash. This is a better test than sticking in a knife- 

 blade, — quicker and more decisive, and more accurate. 



When the copper sulphate is just saturated you have a choice of adding 

 more lime or not. In my opinion, for most purposes it is desirable to add 

 as much lime again as has been added to secure the above test. But the addition 

 of too much lime is not desirable, because you may by this means so dilute 

 or cover up the copper compound as to render it ineffective. 



It will be seen that in saturating the copper sulphate solution with white- 

 wash, it is necessary to keep note of how much whitewash is added. 

 This is done by the use of a quart measure. If no account is kept it will be 

 impossible to tell how much additional whitewash to add. 



The knife-blade test referred to above consists in thrusting a clean knife- 

 blade or other piece of polished iron or steel into the mixture that is being 

 made. If a deposit of copper forms on the iron after a minute or two, the 

 sulphate of copper is not yet neutralised, and more whitewash needs to be 

 added. This test is by no means so sure, or quick, or accurate as that with 

 ferrocyanide of potassium, but it has the advantage of being nearly always 

 readily applied. 



Should too much whitewash be by accident added, so that on the first 

 trial neither of the above tests (copper on the knife-blade or red colour with 

 the ferrocyanide) can be secured, either of two methods can be followed — 



1. Throw the mixture away and start again. 



2. Add more copper sulphate solution until a red colour just begins to 



appear. 



In this latter case, however, it will be essential to know just how much 

 extra sulphate solution is added, so as to know how much to make the 

 mixture up to finally. 



It will probably be best for the beginner to begin in a small way, and if 

 he overshoots the test, throw the mixture away and try again. All the 

 difficulties (and even these are slight) are in learning how ; once learned, 

 this method of preparing the mixture is as easy as any — in fact, considerably 

 easier. 



We will suppose, however, that no accident has occurred, so that the 10 

 gallons of sulphate solution have been properly neutralised with whitewash, 

 and that then as much again whitewash has been added. It now only remains 

 to add water until the whole mixture is made up to 40 gallons, and we 

 have a properly-made Bordeaux mixture containing in every 40 gallons just 

 3 lb. of sulphate of copper. 



