INTRODUCTION. 11 



the appropriate remedy and to use it with discretion. It is the most 

 difficult side of vegetable therapeutics. The therapeutic store contains 

 a great number of products the action of which is analogous. Those 

 which can be usefully employed can be reduced to a small number. 

 The most interesting are carbon disulphide, bouillie bordelaise,^ lime, 

 sulphur, sulphate of iron, sulphuric acid, emerald green, soap-emulsions 

 of petroleum and alcohol, tar, prussic acid, tobacco, and nitrobenzene. 

 The greater number of the chemical products which have been the 

 subject of experiment against the diseases of plants are, nevertheless, 

 dealt with in this treatise, and deductions drawn from the aggregate of 

 the results obtained by the experimenters. The results which have 

 been published are so very different, and the opinions expressed so 

 contradictory, that the author has been obliged to control the facts 

 by personal experiment before expressing aa opinion. Laboratory 

 experiments do not always permit of the conclusion that their results 

 will be confirmed in actual practice ; parasites have natural means of 

 protection which are awanting in the laboratory, but which enable 

 them in a natural state to escape very often from the deadly action 

 of the agents used. Experiments, therefore, to which most weight 

 is attached are those made in practice. According to their mode of 

 action and their nature, chemical agents are used and applied in very 

 different ways. 



Methods of Using Chemical Products in Treating the Diseases 

 of Plants. — Insecticides and anticryptogams are used in three forms : 

 (1) As gas. (2) As powder. (3) In the state of solution or suspension in 

 a liquid vehicle. Use of Chemical Agents in State of Gas. — Gases are 

 used in closed spaces under a cloche'-'- or in the soil. For this pur- 

 pose there is utilized either liquids which evaporate at the ordinary 

 temperature or solid products which disengage gas by heat, combus- 

 tion, or chemical decomposition. In any case it is necessary that the 

 gas mix perfectly with air and reach all the corners of the area to be 

 disinfected. In closed spaces that is comparatively easy, in the soil 

 it is more difficult to realize. 



Underground Treatment. — Injections of volatile liquids are made 

 in the soil at suitable depths by means of an instrument called the pai 

 injector (fig. 5, p. 63), when by sprinkling the soil with water the gas 

 which is disengaged is enclosed. When such treatment is carried out 

 with the necessary care, so as to avoid the contact of the liquid sub- 

 stance with the roots of the plant it yields perfect results. But diffi- 

 culties are encountered due to the nature of the soil. If it be easy to 

 disengage toxic gases in a friable soil it becomes difficult to spread the 

 gases uniformly in a compact wet soil. Gases circulate with difficulty 

 through certain soils, and are not retained long enough in others. 

 Water creates an impenetrable barrier to the circulation of gases. 



1 The translator has retained the original French term throughout. The usual 

 English rendering of the term as " Bordeaux mixture " being, in his opinion, a poor 

 rendering. All bouillies are perforce mixtures, but only a few mixtures are bouillies. 



- Note by Translator. — A bell- or dome-shaped glass vessel familiar to those 

 who dab le in the French style of gardening recently resurrected in Great Britain 

 but well l^nown to London market gardeners at least 150 years ago who even in 

 those early days used them by the hundred. 



