158 DISEASES OF GLASSHOUSE PLANTS 
should be thrown into the next box for sterilization. 
When the boxes are filled they are fitted with lids and 
tarpaulins thrown over the top. The steam is supplied 
from a portable steam boiler of 12 to 30 horse power 
(Fig. 43), and maintained at 60 to 90 lb. pressure per 
square inch. Steam is passed through the grids for 
_ 30 mins., which under normal conditions is sufficient to 
raise the temperature of the soil to 200° F. Any leaks 
of steam round the sides of the boxes must be stopped by 
packing with earth. The passing of steam is continued 
for another 20 to 30 minutes, and the box left on for still 
another 30 minutes. To facilitate operations six boxes 
and four grids should be employed. With a suitable 
head of steam two boxes may be steamed at a time, 
while two are being prepared and two previously steamed 
are cooling. The grids, however, may be pulled out as 
soon as the steam has been turned off. The boxes, being 
smaller at the top than the bottom, are easily lifted off 
the soil—a process performed by two men with lifting 
irons—and the grids being shaped like forks are easily 
pulled out from underneath. A house 250 feet by 26 feet 
can be steamed in 70 hours continuous working. 
(b) ‘‘ Small Grid” Method.—An adaptation of the grid 
system has proved useful in some places (Fig. 42). The 
apparatus, which resembles a comb, is made of 1-inch 
iron tube from 8 to 10 feet long to suit the size of the 
plot to be treated, and to fit the spaces between the pipes 
and walls, etc. The ‘‘ teeth’ are each 2 feet 6 inches 
long, fixed at intervals of 10 inches to 12 inches along the 
“back”? of the grid. Holes a quarter of an inch in 
diameter are made slightly on the lower side of the 
laterals or “‘ teeth,’ and are placed alternately on either 
side at intervals of three inches, so that holes on the same 
side are six inches apart. The end of each lateral is 
sealed, and the last hole should be made exactly on the 
under side of the pipe to allow any condensation water 
to run out. A short connexion of l-inch iron tube 
