6 THE ULTIMATE CONSTITUENTS OF PLANTS 



and adding a solution of barium chloride. A white 

 precipitate of barium sulphate results. 



Sulphur occurs in plants in the form of certain proteins, and in 

 certain organic sulphocyanides, and sulphides, e.g.^ oil of mustard, 

 oil of garlic. 



The detection of nitrogen, phosphorus, and sulphur, as described 

 above, may be carried out with any plants or plant products con- 

 venient at the time, such as wheat grain, mustard seed, grass, 

 potatoes, and from the densities of the precipitates the relative 

 amounts of the elements present in different cases may be roughly 

 compared. 



4. Quantitative Determination of Moisture. 



The substance under investigation has first to be 

 obtained in a suitable state of division to allow of ready 

 loss of moisture under the action of heat. Leaves or 

 stems are chopped finely, while samples of potatoes, 

 beets, etc., are cut into cubes of not more than \ 

 inch edge. A known weight (2 to 3 grams) of the sub- 

 stance is then spread evenly in a porcelain or platinum 

 basin previously heated for half an hour in a steam 

 oven, cooled in a desiccator, and weighed. The basin 

 and its contents are heated for twenty-four hours at 

 100° C. in the steam oven, cooled in a desiccator, and 

 weighed. In order to ensure that all moisture has been 

 lost, it is advisable to continue the heating for another 

 hour and reweigh, and repeat until no further diminu- 

 tion in weight takes place. From the total loss in 

 weight the percentage of moisture is calculated. The 

 dry matter left should be kept for 5, below. 



This method for the determination of moisture is not wholly 

 satisfactory, as in some cases other volatile substances besides 

 water are lost, and, again, certain substances are sometimes present 

 which undergo gradual oxidation {e.g.^ linseed oil in flax seed). 

 When the error resulting from such oxidation processes would 

 be appreciable, it is necessary to perform the experiment in an 



