REFLECTIONS ON DISEASE TREATMENT 161 
ately dry. If it is too wet the steam condenses before it 
has penetrated very far into the soil, and its latent heat 
is lost. Furthermore, it tends to pass quickly through 
the soil in certain definite channels, and the correct 
general diffusion of steam between the soil particles is 
prevented. The result is imperfect sterilization and 
money wasted. 
In the “tray” system the steam is passed into 
inverted shallow trays placed on the soil and from them 
diffuses down into it (Fig. 44). The trays are designed 
to combine lightness with efficiency, and generally are 
made of match-boarding lined with zinc to make them 
air tight. A better type of tray can be made of galvan- 
ized sheet iron reinforced with ‘“‘ T”’ irons and “ angle” 
irons. They vary from 6 to 9 inches in depth and are 
made in various sizes suited to the house to be treated, 
having regard to the position of pipes, purlins, stays, etc. 
The trays are placed on the soil and the steam fed 
from suitable pipes introduced at one side. These pipes 
are laid in shallow trenches a few inches deep, and serve 
to connect one tray with another or with the main steam 
supply pipe. The number of trays treated with steam 
at one time depends upon their size and the amount of 
steam available. Frequently when more than two trays 
are employed at a time it is necessary to pass the steam 
for a long period. The pipe introducing the steam into 
the tray extends to the centre of it, and it is an advantage 
to place a brick close to the end to assist in dispersing the 
steam as it issues from the pipes. Trays 8 feet by 4 feet 
may be steamed in pairs joined by a T-piece, the left half 
of the top of the T-piece passing under one tray, the right 
half passing under the other, and the pipe being con- 
nected to the main pipe. When the pipes and trays are 
in position, the latter are pressed down into the soil, well 
packed round the edges and pipe junctions with earth 
(Fig. 45), and covered with tarpaulins. Steam is then 
turned on, and 14 hours is usually necessary before the soil 
at a depth of 6 inches is raised to a temperature of 200° F. 
II 
