1894.] The Pigments of the Pierithv. 5 



rotation of a fly-wheel. Replacing the porous pots by a transpiring 

 branch, this too maintains the wheel in rotation. This is, in fact, a 

 vegetable engine. In short, the transpiration effects going on at the 

 leaf are, in so far as they are the result of spontaneous evaporation 

 and uninfluenced by other physiological phenomena, of the " sorting 

 demon " class, in which the evaporating surface plays the part of a 

 sink of thermal energy. 



If the tensile stress in the sap is transmitted to the root, the 

 authors suggest that this will establish in the capillaries of the root- 

 surface meniscuses competent to condense water rapidly from the 

 surrounding soil. They show by experiment the power possessed 

 even by a root injured by lifting from the soil, of condensing water 

 vapour from a damp atmosphere. Such a state of things may be 

 illustrated by a system (which the authors realised) consisting of two 

 porous pots connected by a tube and all filled with water ; one, the 

 * leaf," exposed to the air gives out vapour, the other, the " root," 

 buried in damp earth supplies the demand of the " leaf," and an up- 

 ward current in the connecting tube is established. 



III. " The Pigments of the Pieridse. A Contribution to the Study 

 of Excretory Substances which function in Ornament." By 

 F. GOWLAND HOPKINS, Demonstrator of Physiology and 

 Chemistry at Guy's Hospital, London. Communicated by 

 Professor E. RAY LANKESTER, F.R.S. Received October 5. 

 1894. 



(Abstract.) 



The paper deals with the chemistry of the wing pigments of that 

 group of butterflies known as the Pieridee, and demonstrates the 

 excretory nature of these pigments. The following are the salient 

 facts dealt with, most of the statements being based on original obser- 

 vations described in the paper : 



1. The wing scales of the white Pieridae are shown to contain uric 

 acid, this substance bearing the same relation to the scale as do the 

 pigments in the coloured Pieridse, and therefore functioning prac- 

 tically as a white pigment. 



2. The yellow pigment which is so widely distributed in the 

 Pieridae (being found in the majority of the genera) is shown to be 

 a derivative of uric acid. 



3. The properties of this yellow pigment are described, and the 

 results of its analysis are given. The pigments of various yellow- 

 coloured genera are shown to be identical. 



4. It is shown that this yellow pigment may be artificially pro- 

 duced by heating uric acid with water in sealed tabes at high tern- 



