78 STEPHEN HALES 



familiar fact that the vine-stump absorbed water before it 

 began to extrude it. 



He afterwards (pp. 106-7) used a mercury gauge and 

 registered a root-pressure of 32^- inches or 36 feet 5j inches of 

 water which he proceeds to compare with his own determination 

 of the blood-pressure of the horse (8 feet) and of other animals. 

 Perhaps the most interesting of his root-pressure experiments 

 was that (p. 110) in which several manometers were attached to 

 the branches of a bleeding vine and showed a result which 

 convinced him that " the force is not from the root only, but 

 must proceed from some power in the stem and branches," 

 a conclusion which some modern workers have also arrived at. 

 The figure on page 77 is a simplified reproduction of the plate 

 (Fig. 19) in Vegetable Staticks. 



Assimilation. 



Hales' belief that plants draw part of their food from the air, 

 and again that air is the breath of life, of vegetables as well as 

 of animals (p. 148), are based upon a series of chemical experi- 

 ments performed by himself. Not being satisfied with what he 

 knew of the relation between "air" (by which he meant gas) 

 and the solid bodies in which he supposed gases to be fixed, 

 he delayed the publication of Vegetable Staticks for some two 

 years, and carried out the series of observations which are 

 mentioned in his title-page as " An attempt to analyse the air, 

 by a great variety of chymio-statical experiments " occupying 

 162 pages of his book 1 . 



The theme of his inquiry he takes ( Vegetable Staticks, p. 165) 

 from " the illustrious Sir Isaac Newton" who believed that 

 "Dense bodies by fermentation rarify into several sorts of Air; 

 and this Air by fermentation, and sometimes without it, returns 

 into dense bodies." 



Hales' method consisted in heating a variety of substances, 

 e.g. wheat-grains, pease, wood, hog's blood, fallow-deer's horn, 

 oyster-shells, red-lead, gold, etc., and measuring the "air" given 

 off from them. He also tried the effect of acid on iron filings, 



1 It appears that Mayow made similar experiments. Diet. Nat. Biog. s.v. Mayow. 



