201 
ly in the same manner as these changes appear to be 
more slowly effected by the chemical agency of light. 
474. When the electric spark, says M. Cavallo, 
is taken in the tincture of certain flowers, it pro- 
duces effects like an acid, which led some of the 
earlier electricians to consider the electric fluid as an 
acid * 5 an opinion which has been more lately main- 
tained by M. Brugnatelli, who goes so far as to as- 
sert that it actually forms saline compounds by com- 
bining with metals f- But this change of colour does 
not seem to occur when the air is excluded ; for Dr 
Priestley did not observe it to take place in syrup of 
violets, when he electrified it in closed glass tubes J. 
Mr Nicholson and Mr Carlisle found an infusion of 
litmus to be reddened by the positive wire of the 
Voltaic pile, while it remained blue at the negative 
one ; and Mr Cruickshank, who observed the same 
phenomena, supposed the acid, which caused the 
redness, to be the nitric, and the alkali, which gave 
a deeper tint of blue at the negative wire, to be am- 
monia . Dr Wollaston impregnated a card with a 
strong infusion of litmus, and then passed along it, 
by means of two fine gold points, touching the card 
at the distance of an inch from each other, a current 
of electrical sparks. In this state, a few turns of the 
machine were sufficient to produce on the card a 
redness at the positive wire, very manifest to the 
* Ess. on Med. Elect, p. lp. 
f Wilkinson on Galvanism, vol. ii. p. 137 
J Hist, of Elect, p. 703. 
$ Wilkinson on Galvanism, vol. ii. p. 49. 54. 
