mass; it was therefore absolutely necessary in order to set in movement 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 wondering 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 sufficiently 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 repeated Hales' experiments and observations again and again, but forgot that which in his mind bound all the separate facts together.
