86 Britain's Heritage of Science 



oxygen. He then put himself the question, " whether 

 there are not in reality many different substances com- 

 pounded together by us under the name of phlogisticated 

 air ? " and to satisfy himself on that point, he investigated 

 whether the whole of the air could be transformed into 

 nitric acid by combination with oxygen. He found that 

 there was, indeed, a small portion, estimated by him as 

 J-JTJ- of the whole, which resisted the change. This remnant 

 undoubtedly consisted of argon, a separate gas, identified as a 

 new element only in our own times. The amount of argon 

 actually present in the air agrees remarkably well with 

 Cavendish's estimate of his residual gas. 



There are many investigations on heat, unpublished at 

 the time, by which Cavendish anticipated Black in the 

 discovery of latent heat; he also determined the specific 

 heats of a number of bodies. Another important research 

 remains to be noted. A Yorkshire clergyman, John 

 Michell, had conceived the brilliant and ambitious idea of 

 measuring directly the gravitational attraction between two 

 spheres of lead. It has already been remarked, in con- 

 nexion with the Schehallien experiment of Maskelyne and 

 Hutton, that the average density of the earth may be 

 derived from such a measurement, but quite apart from 

 this application, the attempt to demonstrate Newton's 

 gravitational force within the four walls of a room con- 

 stitutes an effort of heroic ambition and remarkable fore- 

 sight. John Michell had constructed all the necessary 

 apparatus, including the torsion balance, which he had 

 invented for the purpose. Infirmities of age prevented his 

 carrying out the work, and at his death the apparatus fell 

 into the hands of another distinguished clergyman, Francis 

 John Hyde Wollaston (brother of the celebrated chemist), 

 who, at the time, held the Jacksonian Professorship at 

 Cambridge. Wollaston deserves considerable credit for 

 handing over the execution of the experiment to the one 

 living man who was capable of bringing it to a successful 

 issue. The original torsion balance consisted of a wooden 

 beam about two yards long, weighing 5| ounces, and 

 carrying at each of its ends a leaden sphere two inches in 

 diameter. Cavendish substituted for the beam a metal rod 



