598 SPECIAL METHODS OF DISCOVERY. 



degrees of intensity of chemical attraction, or that in which 

 they displace each other from their compounds. 



Soon after Klaproth had discovered strontia, Haiiy 

 observed that crystals of ' heavy spar ' from Sicily, and 

 those from Derbyshire (which were considered to be the 

 same substance), differed in their angles of cleavage by 

 3| degrees, and remarked : ' I could not suppose that this 

 difference was the effect of any law of decrement ; for it 

 would have been necessary to suppose so rapid and com- 

 plex a law, that such a hypothesis might have been justly 

 regarded as an abuse of the theory.' French chemists also 

 had found that in consequence of the similarities of the two 

 earths, baryta and strontia, those earths had sometimes 

 been mistaken for each other, and Vauquelin, by chemical 

 analysis had discovered that the base of the crystals from 

 Sicily was strontia, and that of those from Derbyshire was 

 baryta. These facts, becoming known to Haiiy, enabled 

 him by inference to discover, that the angles of crystals 

 might be employed as a test for the presence of different 

 substances which very nearly resemble each other in other 

 respects. 



The atomic theory of chemistry was mainly an inference 

 of Dr. Daiton's, conceived and developed during the years 

 1803 and 1804, and published in 1807 in Thomson's 6 Che- 

 mistry,' and in 1808, in Daiton's ' System of Chemistry.' 

 Lavoisier was one of the first to prepare the minds of 

 chemists for drawing this inference. About the year 1770, 

 he had begun to employ freely the balance in chemical 

 experiments. In 1778, he had shown that 'fixed air (i.e. 

 carbonic anhydride) was composed of definite proportions 

 of carbon and oxygen, viz. 28 parts of the former and 72 

 of the latter ; and that 145*6 grains of iron by combustion 

 in oxygen formed 192 grains of oxide ; he also found that 

 water was a compound of 1 volume of oxygen with about 



