TOO DISEASES OF CROP-PLANTS 



Mercuric chloride must always be treated with the caution 

 due to an exceedingly powerful poison. Metal vessels should 

 not be used to contain the solution. 



Formalin. 



Formalin is the name given to a solution of formaldehyde gas 

 in water, containing, when at full strength, 40 per cent, of the 

 gas. It gives off a vapour which irritates the eyes and throat. 

 Both the solution and the gas are very powerful disinfectants, 

 and are only limited in application by the susceptibility of some 

 seeds and other treated material to injury. They afford the most 

 generally satisfactory means of disinfection. 



In the disinfection of seeds formalin is used at strengths of 

 2 to 0.25 per cent, of the commercial liquid, according to the time 

 of exposure. For prolonged treatment, as of potatoes for scab, 

 or grain for smut, strengths of 0.4 to 0.25 per cent, are used, the 

 thoroughly wetted material being covered up for about two 

 hours. In using the 2 per cent, solution seed is soaked for 15 

 to 45 minutes according to susceptibility. In some cases better 

 results are obtained by long exposure to weak solutions, in others 

 the reverse. A i per cent, solution of formalin contains i oz. of 

 formalin in 5 pints, and from this the other strengths can easily 

 be calculated. 



Certain kinds of seeds cannot be safely treated with liquids, 

 and others are very difficult to wet thoroughly. These diffi- 

 culties can be overcome by the use of formaldehyde vapour, 

 which is in general less injurious and more effective than the 

 liquid treatment. An apparatus has recently been described 

 which is suitable for use where there is regular need for disin- 

 fection of a large variety of seeds. It consists of an arrangement 

 for passing steam, mixed with atomized formalin, into a gal- 

 vanized iron can with perforated shelves. 10-20 ounces of for- 

 malin per 1,000 cubic feet, allowed to act for 2 hours, is about the 

 average quantity required. 



The method of generating the gas commonly used is that of 

 pouring formalin over potassium permanganate crystals at the 

 rate of i imperial pint to 9 ounces. The crystals should be placed 

 at the bottom of a large bucket (for quantities up to 3 pints) and 

 this placed on a tray of some kind, as the mixture foams 

 vigorously. The fumigatorium should be ready for immediate 

 closing when the formalin has been added, or an arrangement 

 should be made for the release of the liquid after closing. 



The above method is also applicable to the fumigation of fruit 

 previous to storage, and has been used with success for pineapples. 



For the disinfection of the soil of seed beds, formalin of 0.5 

 per cent, strength, at the rate of about three quarters of a gallon 

 to the square foot of prepared bed, is distributed with a watering- 

 pot as fast as the soil will take it up. The beds are then covered 

 with bags or tarpaulins for a day or so to keep in the fumes, and 

 are aired for a week before sowing. 



