l50 Tan ViCTOJaiAif >fATUllALiSl'. [Vol. xxlil. 



cussion valueless, for Grew considered that pigments were directly 

 produced by interaction between " sulphur," saline constituents, 

 and the air, green appearing when the air exercises the predomi- 

 nant influence, yellow when the "sulphur" and air are more 

 equivalent, blue when " sulphur " predominates, and red when it 

 is very abundant* (p. 272). The changes of colouration which 

 many flowers undergo in the progress of development were also 

 observed (p. 271), as well as that many roots turn red, green, or 

 purple when exposed above ground. Grew regarded this as due 

 to the direct action of the air, although he was well aware that 

 an ordinary soil is well aerated for some distance below the 

 surface. The fact that water-plants are also able to turn green 

 was regarded as being, due to the large amount of air which such 

 plants usually contain (p. 270). 



The solvent powers of oil, water, and spirits of wine were 

 investigated, and it was observed that olive oil slowly extracts a 

 green and a yellow pigment from ordinary leaves, but extracts a 

 red pigment only from alkanet root. Blue and red pigments 

 were found to be soluble in water, but not green, whereas spirits 

 of wine dissolved the latter readily (p. 274) but not the former; 

 and, moreover, it was noticed that the green extract appeared 

 red by transmitted light, which is probably the first mention of 

 the fluorescence of chlorophyll. Grew also found that blue 

 pigments may turn red on the addition of dilute sulphurous acid 

 and green with ammonia, and that yellows are comparatively 

 unaffected by acids and alkalies (p. 276). From these facts the 

 conclusion was drawn that alkalies predominated in greens, acids 

 in reds, while in blues they approximately neutralized each other, 

 although, as is now known, the observed phenomena have a 

 widely different meaning. 



The work concludes with an account of the smells and tastes 

 of plants (pp. 279, 284), and one appendix is added upon the 

 solution of salts in water (p. 296) and another upon the mercury 

 barometer. Although certain of Grew's ideas upon the latter will 

 hardly recommend themselves to physicists of the present day, 

 still the former is well worthy of attention, for the changes of 

 volume on solution were noticed and the chief phenomena 

 correctly interpreted, while a method is also given for obtaining 

 the specific gravity of a soluble salt. Indeed, it is evident that 

 had Grew turned his energies entirely to physical research his 



* The vague term "sulphur" comprised approximately everything which 

 disappeared when a plant was burnt, and hence included carbon compounds, 

 as well as the actual element sulphur, which was well known to the early 

 chemists. If we suppose that " the influence of the air " comprises not only 

 the action of all gases which may be present in it, but also all influences > light, 

 &c. ) which may reach the plant through the agency of the air, Grew's remarks 

 become more intelligible. 



