CAVENDISH. 101 



fication, whereas it has been found that heat and light 

 are abundantly evolved both by the combustion of 

 metals and sulphur in close vessels by the combustion 

 of hydrogen and azotic gas and by the combination 

 of metals with chlorine ; and also that chlorine, an acid 

 of the strongest kind, contains no oxygen at all, while 

 the alkalis themselves are oxides. The doctrine of 

 latent heat was happily applied by him to the union of 

 gases with bodies, and if he had only followed that 

 doctrine more closely he would have avoided the error 

 into which he fell, and have perceived that other gases 

 as well as oxygen may support flame, and that all, on 

 becoming liquid or solid, must part with heat. Against 

 his error respecting the constitution of acids may justly 

 be set the great merit of his conjecture, that the fixed 

 alkalis are oxides of metals ; for this has been since 

 proved, and the conjecture is a sufficient evidence that 

 he did not doggedly adhere to his theory of the acidi- 

 fying principle. 



It does not appear that Mr. Cavendish ever after 

 1 785, when he discovered the nature of nitrous acid, 

 prosecuted his chemical inquiries so as to make new 

 discoveries ; but beside making numberless useful 

 chemical experiments, about ten years later he engaged 

 in some important experiments upon the force of at- 

 traction. It occurred to him that he could measure 

 that force, and thereby ascertain the density of the 

 earth by accurately observing the action of bodies 

 suddenly exhibited in the neighbourhood of a horizon- 

 tal lever nicely balanced, loaded with equal leaden 

 balls of a small size at its two ends, and protected 

 from all aerial currents by being inclosed in a box. In 

 that box a telescope and lamp were placed, that the 

 motions of the lever might be carefully observed. On 

 approaching the external leaden balls made use of, 

 whose diameter was eight inches, to the small ones 

 inclosed, and near the lever, it was found that a hori- 

 zontal oscillation took place. This was measured; 



