CAVENDISH 147 



Very important was Cavendish's discovery in the year 

 1 78 1, that hydrogen and oxygen when burning together 

 form water, he also showed that the weight of the water 

 formed is equal to the weight of the two kinds of gases which 

 have disappeared. This highly unexpected and astonishing 

 result of the production of water from these newly discovered 

 gases soon became widely known, after Cavendish had first 

 himself communicated it to Priestley; it was the key at last 

 to the composition of water, which had so long been consid- 

 ered, since the ancients, as a simple element. From then on 

 progress was rapidly made in discovering the composition of 

 other bodies, also made up of gases. Cavendish delivered a 

 further important contribution to this, when he showed 

 (1784) that oxygen and nitrogen, mixed together in suitable 

 proportions, disappear when exposed to continual electric 

 sparks, while nitric acid is formed. For this reason, Scheele's 

 fire-air later received the name of oxygen, or 'acid-former,' as 

 being a constituent of acids. It is noteworthy that Cavendish 

 already discovered that a certain proportion of the mixture 

 of air and oxygen remains unchanged even after long passage 

 of the electric sparks. This was the substance only recog- 

 nised as a separate element a hundred years later, and then 

 called argon; a regular constituent of atmospheric air very 

 similar to nitrogen. 



An entirely different kind of investigation of the greatest 

 importance, and one which could only be carried out with 

 the greatest possible skill in the art of quantitative observa- 

 tion, occupied Cavendish's later years, up till 1798; this was 

 the measurement of the earth's density. Stated more 

 correctly, it was not only the first proof, but also measure- 

 ment, of the force of gravity between two terrestrial masses 

 small enough to be contained in an ordinary room. The 

 fact that any two masses exert an attraction upon one another, 

 depending according to Newton's law upon their magnitude 

 and distance apart, was of course, scarcely in doubt after 



