INSECTICIDES. 203 
frequent intervals as the eggs hatch are necessary to control certain pests, 7. ¢., as often as 
twice a week for.a month, if the houses are badly infested. 
The gas is best generated in stone jars which should be distributed at equal distances 
through the house, and not set too close to the plants lest the near ones should receive an 
overdose of the gas and be scorched. 
The jars are dosed with a measured amount of crude sulphuric acid and water (1 to 2), 
and into these are dropped weighed amounts of cyanide of potash wrapped in thin brown 
paper so as to delay the evolution of the gas for a minute, and thus allow the operator to 
escape. To avoid the boiling over of the acid during the violent evolution of the gas, the 
jars should be deep rather than shallow. The house should be shut tight and arrangements 
made in advance to open it from the outside when the exposure is completed. The cyanide 
of potash may also be lowered into the jars from the outside by means of strings; this is a 
rather safer way since the generated gas diffuses through the air with great rapidity, i.e., 
nearly as fast asa man canrun. Sunset of a still day is the best time for commencing the 
exposure. The house should be opened up after 1, 2, or 3 hours. 
The air space of each house must be calculated very carefully and for growing plants 
not more than 0.15 gram of the cyanide of potash should be used for each cubic foot and 
half this quantity for sensitive varieties. Any serious error in the calculation means, of 
course, either failure of the treatment or destruction of the crop. The gas is deadly to man 
and the higher animals, and exposures must not be made in hothouses connected with 
stables or living rooms; and if dwelling houses are near, the doors and windows on that 
side must be closed, or the rooms vacated. The potash salt is also very poisonous and must 
be kept out of the reach of children and animals and handled with rubber gloves. 
A good remedy for red spider is a desideratum. Repeated syringing with water is 
recommended. They are usually worse in dry seasons. 
For the destruction of larve, beetles, and bugs out of doors, a spray containing arsenate 
of lead is effective, and foliage usually bears this poison very well, z.e., much better than 
Paris green. Popenoe used 6 pounds to 50 gallons of water on potato foliage to destroy the 
Colorado potatobeetle. All the larve were killed in 48 hours and the plants were not injured. 
Paris green is also an effective insecticide. It may be sprayed on the foliage, which is 
sometimes burned; or may be dusted on mixed with flour, air slaked lime, or land plaster 
(1 part to 30 or 50). 
Both lead arsenate and Paris green may be combined with Bordeaux mixture, so as to 
avoid two separate sprayings. 
For suggestions respecting trap crops see pp. 282, 295, 296. 
Many of our experiment stations now publish annual spraying calendars and other 
literature, giving the principal formule, and usually these publications may be had upon 
application. 
