EXPERIMENT STATION WORK WITH INSECTICIDES. 249 



proved just as effective in Washington when applied cold as when 

 sprayed hot. The mixture is more difficult to si)ray when cold than 

 when hot. Pear and peach trees sprayed with lime-sulphur mixture 

 when in leaf were quite seriously injured, while apple and plum trees 

 sprayed under the same conditions were very slightly affected. The 

 application of lime-sulphur wash after May 9 in Washington ap- 

 ])eared to be nnich less effective than at earlier dates. A chemical 

 study was made of the changes which take place in the lime-sulphur- 

 salt wash after it is applied to the trees. It was found that the water 

 evaporates, leaving the dissolved solids upon the surface of the 

 sprayed trees. These solids when exposed to the action of the air 

 undergo chemical changes, the sulphids and suli)hites taking up water 

 and being converted into calcium sulphate. The calcium hydrate 

 resulting from the excess of lime ultimately changed into calcium 

 carbonate. These changes take j)lace more rai)idly in a moist than in 

 a dry atmosphere, and certain portions of the deposits are constantly 

 leached out by the solvent action of the moisture in the air. It was 

 found that some suljjhids were still present in the nuiterial on the 

 bark after one week, but had nearly all disappeared by the end of 

 twelve days. The results of these studies indicate, therefore, that 

 the sulj)hur compounds gradually disappear, leaving carbonate of 

 lime as a whitewash upon the bark. 



It is apparent from this sunmiary of work done at various experi- 

 ment stations with lime-sulphur washes that these washes are warmly 

 recommended by entomologists as exceedingly effective in the de- 

 struction of scale insects and certain other insect pests, as Avell as in 

 the control of peach-leaf curl, ai)i)le scab, and some other fungus 

 diseases. The California wash has also the great advantage of being 

 practically harmless to all kinds of trees and shrubs in a dormant 

 condition. Its use is rapidly extending from year to year among 

 horticulturists in all States, but perhaps most rapidly in the peach- 

 growing districts of the South, where it is considered the only remedy 

 which can be safely reconnnended for scale insects. 



PETROLEUM INSECTICIDES. 



Kerosene has been in use as an insecticide considerably longer than 

 lime-sulphur washes. It was first used in the form of an enndsion 

 with soap, sour milk, and various other substances, and later in me- 

 chanical mixtures with water. Petroleum oils have also been used 

 extensively in the place of kerosene in a pure form, in euiulsions, and 

 in mechanical mixtures with water. Considerable attention has been 

 given to the chemical and physical properties of different grades of 

 crude oil obtained from dilferent parts of the country. The crude 

 oils from Pennsylvania, West Virginia, Texas, and California have 



