[ 42i ] 

 XVIII. Curfory Vieiv offome of the latcDifcoveria in Science. 



[Continued from the laft Number, page 309. ~\ 

 LUMINOUS FLUID. 



J- HERE are two principal opinions entertained by phi- 

 lofophers refpetting the nature of the luminous fluid. Some, 

 with the fchool of Epicurus, think that this fluid is a conti- 

 nual emanation of the luminous body which throws to a 

 diftance a portion of its fubftance. This is what is called 

 the emiffion of light, and was the opinion adopted by 

 Newton. 



Others, among whom is Euler, think that the luminous 

 fluid is diffufed throughout infinite fpacc, and that it is acled 

 upon by the luminous bodies as the air is by fonorous bo- 

 dies. This opinion appears to Cit. de la Metherie much 

 more probable than the former. 



One of the ftrongeft objections made to this opinion is, that 

 light is never propagated but in a ftraight line. Objects cannot 

 beffeen, except when no opake body is interpofed between 

 them and the eye, whereas founds are heard in every kind 

 of direction. Euler replied to this, by faying, that bodies are 

 permeable to founds. Befides there are circumftances where 

 founds alf'o cannot be propagated but in a right line ; for 

 example, in echoes which arc heard only in a very fmall 

 fpace. 



Chemifts likewife are divided in their opinions refpec-ting 

 the luminous fluid. Some believe it to be an elementary 

 fubftance, and others have confounded it with fire. Richter 

 believes that it is compofed of the inflammable principle and 

 caloric. 



Benedict Prevoft has endeavoured to eflimate the gravity 

 of the luminous fluid. He made to float on water* tfery thin 

 round plates of tin, and thefe bodies were repelled when a 

 cylinder of red hot iron was prefented to them obliquely at 

 fome ditlancc, or when the rays of the fun collected into a 

 E e 3 focus 



