568 SPECIAL METHODS OF DISCOVERY. 



the pyro-electric properties of crystals of boracite ; and by 

 that method, also, Dr % Wells in 1795, discovered that 

 charcoal is a conductor of electricity. 



Observation constituted an important part of Gralvanf s 

 great discovery, in 1789, of what he termed ' animal 

 electricity.' It is said that he had prepared the legs of 

 some frogs for dissection, and had hung them by copper 

 hooks to an iron balcony ; and as they swayed to and 

 fro by the force of the wind, they touched the iron, and 

 Madame Gralvani observed that when they did so the 

 limbs contracted. Also that his assistant happened to 

 touch with a knife the nerve of a dead frog, just at the 

 time that a neighbouring electric machine was producing 

 sparks, and observed that the limb twitched ; and it is 

 further stated that Galvani soon afterwards discovered by 

 experiment, that contact of two dissimilar metals with 

 the crural nerve and outer muscles of the limb produced a 

 similar effect. As it was previously known to Du Verney 

 in the year 1700, that electric discharges passed through 

 the limbs of a frog, produced muscular contractions, and 

 Galvani had verified this, he concluded that there was 

 ' animal electricity ' in the limbs, which circulated when- 

 ever the nerves and muscles were connected by conduc- 

 tors. It was partly by observation that Cruickshank in 

 the year 1800 discovered that a voltaic current changed 

 the colour of litmus paper. In 1826 Nobili, by the aid of 

 a galvanometer, observed that electric currents were pro^ 

 duced by animal tissues. It was by observing, during the 

 electrolysis of 'water, the proportion of gases evolved in 

 relation to the strength of the current, that Faraday was 

 led to invent the voltameter. The discovery of ' hydro- 

 electricity' arose from an observation made by a man 

 attending a steam-engine at Newcastle, that whenever he 

 touched the boiler he received an electric shock. In 1849 



