MEASURING ELECTRICAL CONDUCTIVITY 37 



displaced to a varying extent when th.e tissue is placed 

 in a solution, with the result that the conductivity is 

 altered. In such cases we must select material in which 

 the displacement is very slow or else we must get rid 

 of the gas at the start by submerging the tissue and evacu- 

 ating by means of an air pump. 



As the writer's investigations were largely concerned 

 with alterations in permeability it was necessary to 

 provide for quick changes of reagents and for rapid 

 penetration. This was accomplished by the use of thin 

 sheets of tissue. For example it was found that when 

 Laminaria was transferred from sea water to sea water 

 diluted with an equal volume of distilled water, 

 diffusion was practically completed in 5 to 10 min- 

 utes ; this was also the case with the other material used 

 in his investigations. 



. It is desirable that the thin sheets of tissue should be 

 stiff enough to be handled easily and that they should 

 not adhere to each other, but should tend to separate 

 spontaneously when the pressure is removed so as 

 to allow a free circulation of liquid between them (this is 

 assisted by choosing pieces with a slight curvature). 



The material should be able to stand laboratory condi- 

 tions and the manipulation required by the experiments. 

 It is desirable that it should be available throughout the 

 year. All these requirements are so admirably fulfilled 

 by the marine alga Laminaria agardhii (a common 

 kelp of the Atlantic coast) that it has been largely used in 

 the investigations of the writer. It forms fronds several 

 feet in length, 3 to 6 inches wide (having somewhat the 

 consistency and thickness of a thin leather belt). It 

 remains in normal condition in the laboratory for several 

 weeks if kept in sea water (near 0°C.) and is not injured 



