10 Methods of Working 



other physiological factor. Consequently great care is needed in inter- 

 preting the results of field experiments without the due consideration 

 of those obtained by other methods. 



II. DETAILS OF METHODS. 



Many details of the sand and soil culture methods have been 

 published by various investigators, e.g. Hiltner gives accounts of sand 

 cultures, while the various publications issued from Rothamsted deal 

 largely with the soil experiments. As this is the case, and as all crucial 

 experiments have always been and must always be done in water 

 cultures, it is only necessary to give here full details of these. 



The great essential for success in water culture work is strict 

 attention to detail. Cleanliness of apparatus and purity of reagents are 

 absolutely indispensable, as the failure of a set of cultures can often be 

 traced to a slight irregularity in one of these two directions. Purity of 

 distilled water is perhaps the greatest essential of all. Plant roots are 

 extraordinarily sensitive to the presence of small traces of deleterious 

 matter in the distilled water, especially when they are grown in the 

 absence of food salts. Ordinary commercial distilled water is generally 

 useless as the steam frequently passes through tubes and chambers 

 which get incrusted with various impurities, metallic and otherwise, of 

 which slight traces get into the distilled water. Loew (1891) showed 

 that water which contained slight traces of copper, lead or zinc derived 

 from distilling apparatus exercised a toxic influence which was not 

 evident in glass distilled water. This poisonous effect was removed by 

 filtering through carbon dust or flowers of sulphur. Apparently only 

 about the first 25 litres of distilled water were toxic, in the later 

 distillate the deleterious substance was not evident. 



The best water to use is that distilled in a jena glass still, the steam 

 being passed through a jena glass condenser. For work on a large scale, 

 however, it is impossible to get a sufficient supply of such water, while 

 the danger of breakage is very great. Experiments at Rothamsted 

 were made to find a metallic still that would supply pure water. While 

 silver salts are very injurious to plant growth it was found that water 

 that had been in contact with pure metallic silver had no harmful 

 action. Consequently a still was constructed in which the cooling 

 dome and the gutters were made of pure silver without any alloy, so 

 placed that the steam impinged upon the silver dome, condensed into 

 the silver gutter and was carried off by a glass tube into the receptacle. 



