THE CELL 37 



It is to him that science owes the first idea of gases, and 

 even the very introduction of the word gas. Not 

 before the end of last century and the development of 

 the chemistry of gases, could the origin of the sub- 

 stance of the plant be fully explained. This explanation 

 followed as a result of the investigations of the three men 

 of science : Priestley, Ingenhouss, and Senebier. 



In order to find out which of the components of this 

 threefold medium — earth, water, and air — participate 

 in the formation of the plant, we must know the com- 

 position of the plant itself. Since Lavoisier, chemistry 

 has taught us that matter not only cannot be created, 

 but in a certain sense does not even change ; that there 

 exist a certain number of so-called simple substances 

 or elements, incapable of transformation one into the 

 other. Therefore, when we find some element present 

 in a plant, we look for it in the surrounding medium, 

 knowing that it must have penetrated thence and 

 could not have been created in the plant, nor produced 

 within it from some other element. 



By no means all the chemical elements are to be 

 found in plants, and even of those which do occur, we 

 shall mention only the principal ones, i.e. those which 

 play a prominent part in the life of the plant. In order 

 to get an idea of the chemical composition of a plant, 

 we submit it to the action of a high temperature. Water 

 evaporates first, and at a temperature a little above 

 ioo° C. we obtain the so-called dry matter of the plant. 

 This is the first step in our analysis. It shows that 

 different parts of a plant contain water in very different 

 proportions (see table on p. 43). At a higher tempera- 

 ture we notice that the dry vegetable matter turns 

 brown and black, and then becomes charred, until it 

 begins to glow and burn with a flame, leaving in the 

 end a heap of ashes, very small in comparison with 

 the quantity of substance with which we started. 

 Most of this substance must therefore have burned 



