CHAP. IT.] of Plants. Hales. 479 



in the wood might possibly sink in the night in consequence 

 of the lowering of the temperature, like the quicksilver in a 

 thermometer, and that so far there might be a return-movement. 

 This was the weak point in Hales' system. 



One of his most important discoveries has generally been 

 overlooked even in modern times, probably because it was 

 entirely neglected by his successors in the i8th century; he 

 was the first who proved, that air co-operates in the building up 

 the body of the plant, in the formation of its solid substance, 

 and that gaseous constituents contribute largely to the nourish- 

 ment of the plant ; consequently that neither water, nor the 

 substances which it carries with it from the earth, alone supply 

 the material of which plants are composed, as had been 

 generally imagined. He showed also with the aid of the 

 air-pump, and better than Nieuwentyt and Wolff, that air 

 enters the plant not only through the leaves but also through 

 apertures in the rind, and circulates in the cavities of the 

 wood. He then connected this with the fact which he had 

 confirmed by numerous experiments, that large quantities of 

 ' air ' are obtained from vegetable substance by fermentation 

 and dry distillation ; the air thus set free by fermentation and 

 heat must in his opinion be condensed and changed to a 

 solid condition during the period of vegetation. He says in 

 chap. 7, that we find by chemical analysis (dry distillation) of 

 vegetables, that their substance is composed of sulphur, volatile 

 salt, water and earth ; these principles are all endowed with 

 mutual power of attraction (of their parts). But air also 

 enters into the composition of the plant, and this in its solid 

 state is powerfully attractive, but in an elastic condition has 

 the highest powers of repulsion. It is on infinitely various 

 combinations, actions, and reactions of these principles that 

 all activity in animal and vegetable bodies depends. In 

 nutrition the sum of the forces of attraction is greater than 

 that of the forces of repulsion, and thus the viscid ductile 

 parts are first produced, and then by evaporation of the water 



