20 S. ]•;. A. McCallan 



Butler (1014, 1923) — as the result of their investigations arc inclined to 

 the view thai various basic sulfates of copper are formed, in addition, of 

 course, to the calcium compounds. The actual basic copper sulfates 

 formed are said to depend largely on the ratio of copper sulfate to lime- 

 water or milk of lime. However, Bell and Taber (1907), the only investi- 

 gators to apply the phase rule to this study, claim that the composition 

 is constant provided a fixed temperature is maintained, and that (page 

 636 of reference cited) the toxic action of the copper must " be attributed 

 either to the solid copper hydroxide (and it is difficult to see how this 

 could act unless in solution), or else to the very dilute copper solution." 



A most important feature is the colloidal nature of bordeaux mixture 

 (Swingle, 1890, and Lutman, 1912, 1916), for it is upon the peculiar blue 

 precipitation membranes that the protective value of this compound 

 rests. 



A good discussion of the nature of copper-lime dust, an approximate 

 equivalent of bordeaux mixture, is given by Boyd (1920). Holland and 

 Gilligan (1928) have presented a general classification of copper fungicides. 



HISTORICAL REVIEW 



A review of the very extensive literature on the liberation of the copper 

 from bordeaux mixture reveals that, in the main, three hypotheses have 

 been advanced to explain the freeing of the copper from the colloidal 

 membranes. These hypotheses are stated by Barker and Gimingham 

 (1911:70) thus: 



1. That copper is brought into solution by atmospheric agencies — more especially by 

 the action of the carbon dioxide of the air, i. c. a purely chemical explanation. 



2. That the leaves, on to which the mixture is sprayed, exert a solvent, action on the com- 

 pounds, i. e. an action of the host plant. 



3. That the fungus itself is responsible for the production of the soluble copper by which 

 it is finally poisoned, i. e. an action of the fungus. 



It is not the intention to present here an exhaustive review of the litera- 

 ture. In general, only those articles containing original experimental 

 evidence in support of these respective hypotheses are summarized. More 

 recent articles are discussed in somewhat more detail. 



ATMOSPHERIC AGENCIKS 



The hypothesis assuming that atmospheric agencies free the copper 

 from bordeaux mixture, has, .without doubt, received the most attention, 

 and perhaps at the present time is the most generally accepted. This 

 hypothesis had its origin in the very development of bordeaux mixture 

 itself, for in 1887 Millardet and Gayon (1887 a: 101) stated (translation 

 of original): "In the first place, it happens that water from rain and 

 dew, in consequence of its carbonic acid and carbonate of ammonia, dis- 

 solves a sufficient amount of copper hydroxide which is on the leaves, 



