28 



happen that such a very poor quality is used that there will be some of 

 the sulphate left unchanged. There are several simple ways by which 

 one can tell when enough lime is present. Those who are very familiar 

 with the reaction which occurs can tell by the color of the precipitate, it 

 having- a greenish* tinge when an insufficiency of lime is present instead 

 of the deep sky-blue color. However those who are not familiar with 

 the process must use more decided tests. Three simple ones can be 

 employed, as follows : 



i. Take some of the clear solution which is left on top when the 

 sediment settles and place in a white saucer. Add a few drops of a solu- 

 tion of potassium ferrocyanide to it. If a reddish-brown precipitate or 

 coloration appears, more lime is needed. 



2. Take a portion of the clear fluid as before, and blow the breath 

 gently over the surface. If a thin white pellicle or covering forms over 

 the top, enough lime has been added. 



3. Take a bright piece of steel, such as a knife blade, and hold it in 

 the mixture for a minute or more. If it becomes coated with copper, 

 more lime is required. 



Test number one is the most reliable and is the one recommended. 



In handling copper solutions use only wooden, brass, and copper 

 vessels ; all other receptacles would be corroded and destroyed by them ; 

 besides, the fungicide itself would be injured. 



Copper compounds are poisonous and therefore should not be left 

 lying around where children or animals can get at them. 



Combined with an Insecticide. Bordeaux mixture is quite often 

 combined with Paris green to impart to it an insecticidal value. In this 

 case the mixture takes the place of water for holding the green in sus- 

 pension. Other recommended arsenicals can be used for this also, such 

 as lead arsenate and 'Calcium arsenite. But if soluble compounds of 

 arsenic are used, such as sodium arsenite, it would be necessary to 

 slightly increase the amount of lime used in making the original Bordeaux 

 mixture. 



Soda Bordeaux. 



This fungicide is made from copper sulphate just as the ordinary 

 Bordeaux mixture. It differs, however, in that caustic soda is used to 

 neutralize the acid property of the blue stone instead of lime ; and that 

 the final mixture contains sodium sulphate instead of calcium sulphate 

 (gypsum). The resulting form of the copper, copper hydrate, is exactly 

 the same, and exerts the same fungicidal power. The reaction which 

 occurs may be represented by the following equation : 



CuS0 4 2NaOH Cu(OH) 2 Na 2 S0 4 . • 



Caustic soda. Copper hydrate. Sodium sulphate. 



The main point in connection with this mixture is that caustic soda 

 is an extremely active alkali and any amount of it added over and above 



* The green precipitate is basic copper sulphate, CuSO., Cu(OH) 2 , which would 

 break up on the leaf under the influence of C0 2 and leave free copper sulphate. 



