WAX-SEAL METHOD FOK DETERMINING WILTING COEFFICIENT. 13 



bottom of the wooden supply tank, so that a complete circulation of 

 water is maintained throughout the whole system. The water level 

 is automatically regulated, since the amount of water delivered by 

 the centrifugal pump is lessened as the head against which the pump 

 operates is increased. The opening from the immersion tank is so 

 situated as to be partly above the level of the water, and all dust and 

 impurities collecting on the water surface are thus swept into the 

 supply tank, leaving the water in the immersion tank perfectly clear. 

 The apparatus as figured is capable of holding 250 glass pots, and has 

 given excellent service in the experiments in which it has been 

 employed. 



As soon as the plants in a pot have been reduced to a wilted condi- 

 tion from which they can not recover when placed in a damp chamber, 



Fig. 1. — Apparatus for preventing sudden changes in temperature during the determinations of the wilt- 

 ing coefficient. The propeller in the central tube causes a constant circulation of the water around the 

 pots. 



the water content of the soil is determined. The soil mass can usually 

 be removed intact by breaking the wax seal around the walls of the 

 pot and gently jarring the edge- of the inverted pot against the bench. 

 The lower two-thirds of the soil mass is taken for the moisture deter- 

 mination, since the roots do not usually develop so extensively in the 

 upper portion. The moisture determination is based upon the loss 

 of water taking place when the soil is dried to constant weight at 

 100° C, the percentage of moisture being based upon the dry weight 

 of the soil. 



COMPOSITION OF THE WAX SEAL. 



For sealing the pots, paraffin, petrolatum, beeswax, and tallow 

 were tried in various proportions. A wax composed of 80 per cent 

 paraffin (melting point 45° C.) and 20 per cent petrolatum was found 



230 



