CHAP, ii.] of Plants. Hales. 481 



mass; it was therefore absolutely necessary in order to set in move- 

 ment and animate this huge mass of attracting matter, that a 

 sufficient quantity of strongly repellent and elastic matter should 

 be mixed with it ; and since a large portion of these elastic 

 particles are constantly changing to a solid condition through 

 the attraction of the other parts, they must be endowed with 

 the power of again assuming their elastic condition, when they 

 are set free from the attracting mass. Thus the formation and 

 dissolution of animal and vegetable bodies go on in constant 

 succession. Air is therefore very important to the production 

 and growth of animals and plants in two ways ; it invigorates 

 their juices while it is in the elastic state, and contributes 

 much to the firm union of the constituent parts, when it has 

 become fixed. 



We see what good use Hales could make of the small stock 

 of ideas in physics and chemistry at his disposal, and that he 

 succeeded with their help in rising to a point of view, from 

 which he was able to form some idea of the phenomena of 

 vegetation in their most important relations to the rest of 

 nature, and in their inner course and connection. But his 

 successors did not comprehend the fundamental importance of 

 these considerations, and made no use of the pregnant idea, that 

 a much larger part of the substance of plants comes from the air 

 and not from the water or the soil; they were for ever wonder- 

 ing that so little is furnished by the soil to the plant, as Van 

 Helmont had shown, though they did not confess to supposing 

 that the water was changed into the substance of the plant, as 

 he had imagined. Thus physiologists lost sight of the principle, 

 which might long before the time of Ingen-Houss have suffi- 

 ciently explained the most important of all the relations of the 

 plant to the outer world, namely that it derives its food from 

 the constituents of the atmosphere, and so neglected further 

 experimental enquiry into the matter ; they quoted and re- 

 peated Hales' experiments and observations again and again, 

 but forgot that which in his mind bound all the separate facts 

 together. i i 



