June 2, 1911] 



SCIENCE 



867 



the absorption of the arsenical by the sprayed 

 plants. It is assumed that solid bodies can 

 not penetrate the epidermis of healthy plants, 

 but the absorption of liquids is known to take 

 place. Nothing has developed to show that the 

 first premise is unsound, and the question 

 may well be asked, why have many of the 

 arsenical compounds designated as insoluble 

 by chemists, proved to be injurious when ap- 

 plied as sprays? The most obvious answer is 

 found in the supposition that these com- 

 pounds are not insoluble under the conditions 

 of the application. 



Solubility implies a solvent, and the most 

 universally present solvent found in nature is 

 water. Chemists base their estimates on solu- 

 bility in pure water, and on the results of 

 comparatively short exposures to this solvent. 

 On the other hand, natural water may not be 

 pure, and under some climatic conditions the 

 exposure of spray deposits, on plants, to the 

 action of natural water may be very pro- 

 longed. 



It is very important to note that the last 

 implied condition, that of prolonged exposure 

 of the spray deposit to moisture, is one which 

 prevails to a great extent in the Pajaro Val- 

 ley, California. This valley is situated near 

 the coast, about one hundred miles south of 

 San Francisco, and opens out onto the ocean. 

 The apple orchards of the locality have ex- 

 hibited a remarkable susceptibility to arsenical 

 injury, and to such an extent as to seriously 

 interfere with effective control of the codling 

 moth by the use of arsenic compounds. The 

 industry is a large one, as the production has 

 averaged over three thousand ears annually 

 for the last ten years. An industry of such 

 proportions was deserving of considerable at- 

 tention from those delegated to foster the hor- 

 ticultural interests of the state, and the uni- 

 versity experiment station has properly re- 

 sponded to the demand. 



Field and laboratory work was commenced 

 in the spring of 1903, and the author became 

 connected with the investigation in the fall of 

 that year. Since that date the work has gone 

 on continuously, and largely under the 

 writer's supervision. 



That the results have been satisfactory, is 

 reflected by the methods of spraying and ma- 

 terials used by the growers at the present 

 time. The influence of this investigation is 

 also apparent in the orchard practise of the 

 entire Pacific coast and many of the interior 

 states, but comparatively little publication has 

 been done. Now that the attention of many 

 investigators is being attracted to arsenical 

 injury and kindred problems it appears that 

 the results of our work should be properly 

 published. With this end in view, the author 

 has recently issued a circular entitled " Foli- 

 age Tesiing of Arsenicals." In the present 

 article it is intended to cover in more detail 

 the purely chemical considerations. 



To continue with water solubility; the cli- 

 matic conditions of the Pajaro VaUey are 

 peculiar in that there is a large amount of 

 fog and dew moisture through the spring and 

 summer months. The foliage of the trees 

 becomes wet early in the evening and remains 

 so during the night and well into the follow- 

 ing morning. This condition may continue 

 without interruption for a period of several 

 weeks. The dew and fog moisture may at 

 times be abundant enough to drip liberally 

 from the trees, but more often it is nearly all 

 retained on the foliage where it evaporates 

 during the following forenoon. Such condi- 

 tions are evidently ideal for dissolving sub- 

 stances on the leaves, and for the absorption 

 of the solutions by the plant tissues. As con- 

 trasted with rains which wash dissolved sub- 

 stances entirely away, the above-described 

 conditions are much more trying, and partly 

 explain why apple foliage in the eastern states 

 has been so little subject to arsenical injury. 



With regard to the action of substances in 

 solution in the water used to suspend arsen- 

 ical compounds, when applied as a spray, Hay- 

 wood and McDonnell' and others have shown 

 that chlorides, carbonates and sulphates render 

 the arsenic oxide in certain varieties of ar- 

 senate of lead more soluble and increase the 

 danger of foliage injury. On the other hand, 

 we have found that distilled water does not 



' XJ. S. Dept. of Agr., Bureau of Chemistry, 

 Bui. 131, p. 46. 



