CHAP, ii.] of Plants. Hales. 477 



measurements and calculations combine to form a living 

 picture of the whole subject. Malpighi endeavoured to dis- 

 cover the physiological functions of organs by the aid of 

 analogies and a reference to their structure; Mariotte discerned 

 the main features of the connection between plants and their 

 environment by combining together physical and chemical 

 facts ; Hales may be said to have made his plants themselves 

 speak ; by means of cleverly contrived and skilfully managed 

 experiments he compelled them to disclose the forces that 

 were at work in them by effects made apparent to the eye, and 

 thus to show that forces of a very peculiar kind are in constant 

 activity in the quiet and apparently passive organs of vegetation. 

 Penetrated with the spirit of Newton's age, which notwith- 

 standing its strictly ideological and even theological conception 

 of nature did endeavour to explain all the phenomena of life 

 mechanically by the attraction and repulsion of material 

 particles, Hales was not content with giving a clear idea of the 

 phenomena of vegetation, but sought to trace them back to 

 mechanico-physical laws as then understood. He infused life 

 into the empirical materials which he collected by means of 

 ingenious reflections, which brought individual facts into 

 connection with more general considerations. Such a book 

 necessarily attracted great attention, and for us it is a source 

 of much valuable instruction on matters of detail, though we 

 now gather up the phenomena of vegetation into a somewhat 

 differently connected whole. 



His investigations into transpiration and the movement of 

 water in the wood were greeted with the warmest approbation. 

 He measured the quantity of water sucked in by the roots and 

 given off by the leaves, compared this with the supply of 

 moisture contained in the earth, and endeavoured to calculate 

 the rapidity with which the water rises in the stem, and to 

 compare it with the rapidity of its entrance into the roots and 

 its exit by the leaves. The experiments, by which he showed 

 the force of suction in wood and roots, and that of the root- 



