36 THE LIFE OF THE PLANT 



metaphysics, at once a mystic and an ingenious experi- 

 menter Van-Helmont, I say, made the first exact 

 experiment, which tended towards the solution of the 

 problem of the origin of the substance of the plant. 

 This experiment is remarkable not only because it is 

 the first exact experiment in the province of plant 

 physiology, but also because it was among the first 

 cases in which a balance was used as a means for solving 

 a problem in chemistry. It is well known that chemistry 

 owes to Van-Helmont the original application of this 

 instrument, which later on, in the hands of Lavoisier, 

 revolutionised that science. Let us describe Van- 

 Helmont's experiment in his own words. ' I placed,' 

 he says, ' two hundred pounds of earth, previously 

 dried in an oven, in an earthenware pot and planted a 

 willow slip in it, weighing five pounds. Within five 

 years the willow slip weighed one hundred and sixty-nine 

 pounds, three ounces. The pot was regularly watered 

 with rain and distilled water. The pot was large, and 

 buried in the soil ; and, that it might be protected from 

 dust, it was covered with perforated tin foil. I did not 

 weigh the leaves shed by the plant during the four 

 successive autumns. At the end of the five years I 

 redried the earth and found that it weighed the same 

 amount of two hundred pounds minus two ounces, 

 which meant that water alone had been sufficient for 

 the production of one hundred and sixty-four pounds of 

 wood, bark, and roots ' (Ortus medicinae, p. 109). This 

 experiment proved beyond doubt that earth or rather 

 soil cannot be considered the exclusive or even the 

 chief source of vegetable matter. Van-Helmont saw it 

 in the water he used for watering the plant ; we know, 

 however, that the plant derives its substance not only 

 from earth and water but also from the air. Neverthe- 

 less, Van-Helmont's inference was perfectly correct as 

 far as he could go. In his day science had no definite 

 conception of the third, i.e. the gaseous, form of matter. 



