144 BIOLOGICAL PROBLEMS 



lardet took the matter up, and published results in 

 1886. About a year later mixtures were devised 

 wherein washing-soda was used instead of lime. The 

 use of these washes rapidly spread throughout Europe, 

 and they soon became recognized as fungicidal reme- 

 dies of the utmost value. No time was lost in adopt- 

 ing Bordeaux mixture to combat potato blight, and 

 later it was used against various fungal pests of 

 fruit trees in the United States, Canada, Australia, 

 and elsewhere. 



Burgundy mixture is now more generally used, 

 since it can be made of more constant composition, 

 and free from gritty particles, which choke the nozzle 

 and exert a wearing effect on the working parts of the 

 spraying machine. 



Burgundy mixture is produced by the interaction of 

 copper sulphate and washing-soda, and only standard 

 chemicals of guaranteed purity should be used. Copper 

 sulphate is a virulent plant poison, so much so that a 

 i-per-cent solution is used for charlock-killing, and 

 a i-per-cent solution of this sort alone would prove 

 deadly to the potato. The copper sulphate is just 

 neutralized when the chemicals are mixed in the 

 proportions of their molecular weights, but it is better 

 to use an excess of soda to obviate the risk of obtain- 

 ing free copper sulphate. 



The product of the reaction, copper carbonate, 

 varies in physical qualities according to conditions; 

 for example, in adhesiveness, specific gravity, and 

 texture. The best results are obtained when the 

 chemicals are mixed in cold solution, forming a more 

 adhesive compound at lower temperatures, and when 

 the solution of soda is added to a weak solution of 

 copper sulphate, since in this case the precipitate is 

 more flocculent and evenly distributed throughout the 

 liquid. 



