Rothamsted Symposium on Trace Elements 14 



for normal growth of most plants. The sand is retained in the 

 pots by a layer of glass wool covered with a glazed porcelain filter 

 disc or a small watch glass. These can be cleaned in acid before 

 use. 



Bitumen painted clay pots, however, are not suitable for the 

 study of molybdenum deficiency and for this purpose five litre 

 capacity pyrex glass beakers provided with a central drainage hole 

 one inch diameter are most successful. 



Certain plastic materials, notably polythene, are useful for 

 making large containers or pot linings and polythene has proved 

 suitable for work involving deficiencies of iron, manganese and 

 boron. Care, however, must be taken to avoid the presence of 

 harmful plasticising compounds. Contamination from catalysts 

 and during manufacture may also occur and require consideration 

 when plastics are used. Paraffin wax linings were found unsatis- 

 factory owing to biological breakdown of the wax. Attempts to 

 use wax-soluble organic mercury compounds to prevent this re- 

 sulted in stimulation and increased fresh weights of crops grown 

 in such experiments. 



Sand:— The principal impurity in the sand is iron which exists 

 mostly as ferric oxide concretions and as a stain on the surface of 

 the grains and often occurs in minute crevices. Impurities in 

 lesser amounts include traces of manganese, copper, zinc, boron 

 and molybdenum, all of which occur mainly in the larger frag- 

 ments present in the sand. The sand is purified first by removing 

 the fraction containing these fragments by sieving on a 24 mesh 

 pure copper gauze, and then treating with concentrated hydro- 

 chloric acid containing oxalic acid. 



As several tons of sand must be treated, this acid process is 

 carried out automatically in the apparatus illustrated in Figure 2. 

 It is constructed in an acid resisting and inert material called "Kee- 

 bush" and comprises a vessel with a conical base and a central 

 tube with apertures around the base. A steam jet projects a short 

 distance beyond the holes. The vessel is filled with sand satu- 

 rated with acid, and steam is forced through the jet. This carries 

 the sand and acid up the central tube and circulates the whole 

 mixture whilst at the same time heating it strongly. The ap- 

 paratus is automatic and a batch of 250 pounds of sand may be 

 treated for several hours unattended. The treated sand is leached 

 in large glazed vessels first with water and then with suitably 



