12 INSECTICIDES, FUNGICIDES, AND WEED KILLERS. 



The gases produced in the soil should not enter into reaction therewith 

 and be fixed by the substances in the soil. From this point of view 

 carbon disulphide is the best substance. Other substances, such as tar, 

 petrol, benzene, sulphuretted hydrogen, are retained by the capillarity 

 and chemical action of the soil which often energetically opposes their 

 distribution. To avoid failure it is well to give the preference to 

 insecticides in dilute solutions in winter, and to volatile insecticides 

 in summer, when the soil is dry. 



Aerial (or above ground) Treatment by Gases. — Clochage, or 

 treatment in a closed space, gives the most certain results, and does 

 not exert an unfavourable influence on the development of the plant. 

 Highly poisonous gases may be used against parasites, because they do 

 not generally have a deadly action on the plant, especially when con- 

 tact with the plant is not prolonged beyond measure, which result is 

 obtained by aerating after a predetermined time. 



Clochage is used to disinfect the vine by sulphurous acid. The 

 stock is covered with a cloche made of a tun (cask) cut through the 

 middle, or with a zinc receiver fitted with two handles. Under the 

 cloche the gas is disengaged by combustion or by chemical decomposi- 

 tion of certain salts : sulphur is burnt, or potassium cyanide is decom- 

 posed by sulphuric acid. The operation is finished in ten minutes. 

 In greenhouses or in closed spaces made around fruit trees or against 

 espaliers with waterproof awning, the operation is performed in the 

 same way. In all cases where disinfection by gas is possible, it ought 

 to be applied as a process sure to disinfect without injuring the plant 

 treated. It is the only process applicable to food warehouses. Treat- 

 ment by gas is always curative. When this treatment is not applicable 

 recourse is had to treatment by boiling water, or solutions of toxic 

 substances, emulsions, or pulverulent products. 



Scalding or treatment by boiling water finds a very extended 

 use in winter to kill by heat all parasites and their germs lodged 

 along the trunk of a plant. But that is a winter treatment which 

 cannot be applied in summer, the delicate organs of the plant not 

 being able any more than the parasites to support contact with hot 

 water. 



Use of Chemical Agents in the Form of Powder. — Non- 

 poisonous but asphyxiant powders are used such as they are ; toxic 

 powders are reduced more or less according to the intensity of their 

 insecticidal or sporicidal capacity with flour, talc, chalk, or any other 

 inert matter, finely divided and cheap. The powders are projected on . 

 to the plant by means of bellows called sulphurators (figs. 3 and 4, 

 p. 47). Powders may be projected where liquids cannot penetrate. 

 Liquid treatments are sometimes alternated in the struggle against 

 stubborn diseases with pulverulent treatments of the same composition. 



Use of Chemical Products in the Liquid Form. — Poisonous 

 substances in a state of solution are used in both the external and 

 internal treatment of plants. In the external treatment the poisonous 

 substance is spread on the plant, whilst in the internal treatment it is 

 introduced into the juice, either by causing it to be absorbed by the 

 roots or by injecting it into the trunk. External treatment is most 



