LIB. II. 35, 36. ^37 



nino esse divisio vera et certa, lucis, qua? est visibile 

 originate, et primam copiam facit visui ; et colons, qui 

 est visibile secundarium, et sine luce non cernitur, ita 

 ut videatur nil aliud esse quam imago aut modificatio 

 lucis. Attamen ex utraque parte circa hoc videntur 

 esse instanticK fcederis ; scilicet, nix in magna quanti- 

 tate, et flamma sulphuris : in quarum altera videtur 

 esse color primulum lucens, in altera lux vergens ad 

 colorem. 



XXXVI. 



Inter prasrogativas instantiarum ponemus loco de- 

 cimo quarto instantias crucis ; translato vocabulo a 

 crucibus, qua?, erecta? in biviis, indicant et signant via- 

 rum separationes 6 . Has etiam instantias decisorias, et 



6 Instances of the Cross, or Cru 

 cial Instances, are the most fa 

 mous of all, and of the great 

 est value in Physical Investigation. 

 &quot; They afford the readiest and se 

 curest means of eliminating extra 

 neous causes, and deciding between 

 rival Hypotheses.&quot; Herschel s Dis 

 course, 196. &quot; When,&quot; says 

 Playfair, Diss. iii. p. 465, &quot; in in 

 vestigation, the understanding is 

 placed in equilibria, as it were, be 

 tween two or more causes, each of 

 which accounts equally well for the 

 appearances, as far as they are 

 known, nothing remains but to 

 look out for a fact which can be 

 explained by one of these causes, 

 and not by the other : if such an 

 one can be found, the uncertainty 

 is removed, and the true cause de 

 termined.&quot; Chemistry is full of 

 these Instances. Playfair gives the 

 famous illustration of the escape of 

 &quot; Phlogiston&quot; or &quot; Absolute levity&quot; 

 from metals when calcined. As it 

 was seen that, when calcined, metals 

 weighed more than they did before 



the application of Heat, it was ne 

 cessary to account for the fact. 

 There would be two theories: (i) 

 that of Phlogiston the escape of 

 &quot; absolute levity.&quot; (2) The intro 

 duction of some new chemical 

 compound. Lavoisier exploded the 

 Phlogistic theory by a Crucial In 

 stance. He took a quantity of Tin, 

 sealed it hermetically in a glass re 

 tort, and then weighed the glass 

 with its contents. Then he cal 

 cined the metal, and weighed the 

 glass again. It was found to have 

 suffered no change of weight. 

 When it was cool, the retort was 

 opened, and air rushed in, shewing 

 that a partial vacuum had been cre 

 ated. The glass was then weighed 

 a third time, and it had now gained 

 ten grains ; i. e. ten grains of air 

 had rushed in. The calcined Tin 

 was then taken out, and was found 

 to be ten grains heavier than before 

 the application of fire. These ten 

 grains had, therefore, been absorbed 

 from the air. This experiment led 

 to the discovery of Oxygen Gas, 



