203 
to the change of colour in the vegetable infusion, 
since its nitrogen contributed to the formation of ni- 
tric acid. In the experiments of Brugnatelli, this 
acid, also, was probably produced ; and either the 
formation of acid and alkali, or their development 
by the decomposition of saline matter, is clearly in- 
dicated in the experiments of Nicholson and of 
Cruickshank. In Dr Wollaston's experiment, the 
acid and alkali were probably derived from saline 
matter, existing either in the infusion with which the 
card was impregnated, or in the card itself ; for Mr 
Davy remarks, that the minute quantity of saline 
compound, contained in paper tinged with turmeric, 
and which has been plunged in pure water, affords 
alkaline matter at the negative point of the circuit ; 
and acid, in the same manner, is developed from lit- 
mus paper, at the positive point *. All these facts, 
therefore, afford striking illustrations of the great fa- 
cility with which acid and alkaline matter may be 
generated, or developed, by the agency of electrici- 
ty ; and of the capacity of these substances, when 
thus produced, to act on the colourable juices of ve* 
getables. 
477. Nor is it only on these juices, after their ex- 
traction from the plant, that electricity is found to 
act ; for results, in some degree similar, follow the 
operation of the same agent, when applied to the 
living leaf. Dr Priestley found, that electric explo- 
sions, passed over a green leaf, injured its texture, 
and left a yellow track behind f ; which effect was 
* Phil. Trans. 1807- t Hist. Elect, p. 684. 
