1887 .] Dr A. B. Griffiths on Rays of Solar Spectrum. 
127 
The soil used in the fourteen experiments 
was a calcareous soil of the 
following composition : — 
Lime, ...... 
53-00 
Organic matter, .... 
1-50 
Oxides of iron and alumina, 
... 1 -62 
Magnesia, ...... 
0-32 
Potash and soda, .... 
o-oi 
Phosphoric acid, .... 
0-04 
Silica, ...... 
0-26 
Chlorine, ..... 
o-oi 
Sulphuric acid, .... 
None. 
Carbonic acid, .... 
43-24 
100-00 
Graphic Representation of the Amount of Ferric Oxide in the Ashes 
of Plants after growing in an Iron 
Manure, and exposed to 
various Rays of the Spectrum, illustrating Root- Absorption. 
These experiments therefore show that the most active rays for root- 
absorption coincide with those of assimilation. From the researches 
of Drs Draper and Pfeifer, and those of Cloez and Gratiolet, it is 
evident that the most favourable rays of white light are those lying 
between the yellow and green. In their experiments the greatest 
amount of oxygen was evolved in this part of the spectrum, there- 
fore the largest amount of carbon is retained by the process of 
assimilation. From our experiments we obtain the largest per- 
centage of ferric oxide in the ashes of those plants exposed to the 
yellow or yellow-green part of the spectrum, or between Fraunhofer’s 
