50 SOME RECENT RESEARCHES IN PLANT PHYSIOLOGY 



THE DETERMINATION OF MOISTURE AND 

 THE PREPARATION OF ANHYDROUS SUBSTANCES. 



Iii^xnaking out tables of the reducing-powers of sugars, 

 it has been usual to dry the material to be tested in a 

 desiccator over sulphuric acid, and to introduce a correc- 

 tion for the small amount of moisture still retained in it. 

 The latter is only removed after special treatment, such as 

 heating in vacuo to temperatures from 60 to 110, accord- 

 ing to the substance being dried, and by using phosphorus 

 pentoxide to absorb the water vapour. It is, however, 

 obviously preferable that the whole sample should be an- 

 hydrous, as, especially with crystalline substances, it is 

 never quite certain that the moisture is uniformly dis- 

 tributed. For example, sodium chloride, when prepared 

 as a fine dry powder, crackles when heated, owing to the 

 inclusion in the minute crystals of still more minute drop- 

 lets of water. In this case the moisture can of course be 

 removed by fusing the salt, but with many organic sub- 

 stances even prolonged drying in air at 100 results in 

 oxidation to a more or less serious extent. 



To remove water under conditions which preclude oxida- 

 tion and the destruction of thermolabile substances, Shakell 

 (1909) recommends the freezing of the tissue or extract, 

 and its evaporation as ice under reduced pressure. This 

 method appears to be a very perfect one for drying the 

 material sufficiently to prevent bacterial action, and it may 

 even be possible to obtain anhydrous tissues in this manner, 

 though, so far as the writer is aware, Shakell's aim was only 

 to preserve the constituents of serum and antitoxins un- 

 altered. 



When dealing with substances which are not injured by 

 heating to 80 in absence of air, Atkins and Wilson 

 (1915), however, have found it possible to remove water 



