36 ESSAYS BIOGRAPHICAL AND CHEMICAL 



vitriolatum, will dissolve 33 of marble; as I found by 

 experiment that so much oil of vitriol would saturate as 

 much fixed alkali as a quantity of nitrous acid sufficient 

 to dissolve 33 of marble.' Richter's tables were published 

 in 1792. Cavendish's remarks involve a knowledge of 

 fixity of proportion, and also of reciprocal proportions; 

 doctrines which were after nearly twenty years pro- 

 pounded by Dalton. 



Perhaps the most important piece of physical work 

 ever performed was Cavendish's determination of the 

 constant of gravitation, or as it is often called, ' the weight 

 of the earth.' The experiment is usually spoken of as 

 the 'Cavendish experiment,' although the method of 

 executing it was first suggested by the Rev. John Mitchell. 

 A delicate torsion balance, suspended by a wire, had 

 leaden balls suspended at each end. Two heavy spherical 

 masses of metal were brought near the balls, so that their 

 attraction tended to draw the two balls aside. The de- 

 viation of the arms was observed, or calculated from the 

 time of vibration ; and from the data found, it is easy to 

 calculate the attraction of a sphere of water, equal in 

 mass to the ball or a similar ball resting on its surface ; 

 and so to determine the density of the earth, knowing the 

 attraction which it exerts on the ball. The results 

 obtained compared very favourably with the best results 

 obtained by other observers, using the utmost precau- 

 tions ; and it is a very remarkable instance of Cavendish's 

 experimental skill and ingenuity. 



We have here to consider more particularly Caven- 

 dish's chemical work. It was of the highest order, and 

 bears the imprint of a master mind, guiding a master 

 hand. 



Before Black's time, the word * gas ' had no plural. 

 Indeed, what we now know as a gas was set down as a 

 modification of ordinary air. Black, however, proved that 



