PHYSICAL AND LITERARY; 81 



Vefle], is rolled out of its due pofition ? Orj 

 if it retains a fixed fituation in the head, is 

 the perception of the (hip's motion owing to 

 a vertigo in the brain, a deception of the 

 imagination ; or to what other caufe ? 



QuER. XVII. Has not gold been reduced, 

 by beating, to a degree of thicknefs Httle 

 exceeding that which muft be afcribed to its 

 colorific parts, according to Sir Ifaac New- 

 tons theory ? But, how can it cohere into a 

 continuous leaf, fo as to leave no vifible 

 pores, unlefs there be many of its compo- 

 nent particles contained in its thicknefs ? 



QuER. XVIII. When one looks ftedfaft- 

 ly at Sirius or any bright flar not much ele- 

 vated above the horizon, its colour feems not 

 to be conftantly white, but appears tindtured, 

 at every twinkling, alternately with red and 

 blue : To what is this appearance owing ? 

 Is not the feparation of colours by the re- 

 fraction of the atmofphere too fmall to be 

 perceived ? 



Qu_ER. XIX. Bodies become black by 

 burning; becaufe they are reduced* into 

 very fmall parts : but, whence is it, that 



Vol. II. L moft 



* Ne-wti Opt. Book iii. part 2. prop. 7. 



