CAVENDISH. 441 



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 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 

 1785, when he discovered the nature of nitrous acid, 

 prosecuted his chemical inquiries so as to make 

 new discoveries ; but beside making numberless use- 

 ful chemical experiments, about ten years later he 

 engaged in some important experiments upon the 

 force of attraction. 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 

 horizontal lever nicely balanced, loaded with equal 

 leaden balls of a small size at its two ends, and pro- 

 tected 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 ob- 

 served. 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 



