600 SPECIAL METHODS OF DISCOVERY. 



equal volumes of all substances, when in the gaseous state, 

 and at the same temperafure and pressure, contain the same 

 number of molecules ; and this inference has been largely 

 confirmed by the labours of subsequent investigators. 



Many discoveries have arisen from drawing similar 

 inferences from similar facts ; that of the artificial for- 

 mation of alizarine arose in this way. ' A short time 

 since, Grraebe, a German chemist, in investigating a class 

 of compounds, called the quinones, determined incidentally 

 the molecular structure of a body closely resembling aliz- 

 arine, which had been discovered several years before. This 

 body was derived from napthaline, and, like many similar 

 derivatives, was reduced back to napthaline when heated 

 with zinc-dust. This circumstance led the chemist to heat 

 also madder alizarine with zinc-dust, when, to his surprise, 

 he obtained anthracene. Of course, the inference was at 

 once drawn, that alizarine must have the same relation to 

 anthracene that the allied colouring matter bore to nap- 

 thaline, and, more than this, it was also inferred that the 

 same chemical processes which produced the colouring 

 matter from napthaline, when applied to anthracene, 

 would yield alizarine. The result fully answered these 

 expectations, and now alizarine is manufactured on a 

 large scale from anthracene obtained from coal-tar.' l 

 The discovery of one hydrogen acid, viz., hydrochloric, 

 enabled the inference to be drawn, and helped the dis- 

 coveries to be made that hydrofluoric, hydrobromic, and 

 hydriodic acids were acids of hydrogen ; Ampere was the 

 first to discover by inference that hydrofluoric acid was a 

 hydrogen acid. 



In the concrete as well as in the simple sciences, 

 inference has evolved many hypotheses and discoveries. 

 Linnaeus, in the early part of the eighteenth century, was 



1 Cooke, The New Chemistry, p. 319. 



