OIDIUM OR POWDERY MILDEW OF THE VINE. 345 



a small pinch on the glass, wet it with a few drops of ether, and shake. 

 This will distribute the sulfur in a thin layer over the glass and much 

 facilitate the examination. 



5. A test of the purity may be made by burning a pinch of the 

 sulfur on a piece of white porcelain a broken plate, for example. To 

 make the burning complete, hold the piece of plate over a lamp or on 

 top of a very hot stove. A pure sulfur burns away completely, leaving 

 only a black stain on the plate. Any dust or particles that are left 

 indicate adulteration. 



The fine sulfurs are superior in three important respects: (1) 

 They capable of more perfect distribution; (2) they offer a larger 

 surface to the air and therefore give off more fumes; (3) they adhere 

 better to the leaves. 



A pound of a very fine sublimed sulfur such as No. 30 (see fig. 13), 

 the particles of which measure .01 mm. in diameter, if evenly distrib- 

 uted and none lost, would give about 15,000 particles for every square 

 inch of leaf and cane surface on an acre of large vines. A pound of a 

 very coarse sulfur such as No. 8 (see fig. 16), the particles of which 

 have twenty-five times as large a diameter, would supply only about 

 one particle for every square inch. Moreover, a much larger propor- 

 tion of the coarse sulfur would be lost by failing to adhere to the leaves 

 and falling on the ground. Perhaps the most important advantage of 

 the finest sulfur in this respect is that the impalpable powder floats so 

 lightly in the air that it reaches and adheres to the lower surfaces of 

 the leaves and canes. A finely powdered sulfur such as No. 16 (see 

 fig. 15) would probably be just as effective as the sublimed, for though 

 it contains a certain proportion of coarser particles, the finer material 

 has not the tendency to cohere in the groups of particles characteristic 

 of sublimed sulfur. 



The quantity of fumes given off by a sulfur must be about in the 

 proportion of the extent of surface which the particles present to the 

 air. This surface is inversely proportionate to the diameter of the par- 

 ticles. A pound of sulfur, No. 30 or No. 16, would possess a surface 

 twenty-five times as great as that of No. 14, and would therefore give 

 off fumes much more rapidly and in greater quantity. 



OTHEE METHODS OF TREATMENT. 



Combined Treatments. Various other substances and sprays have 

 been suggested and tried for the control of the Oidium during the 

 growing season. There are two cases in which it has seemed desirable 

 to attempt to find another method of control. One of these is where it 



