G 



G 



which are diftinguifhed by phe curious in botany, , twenty-five years to pafs from the earth to the furt, as 

 but as they are plants of no ufe, I {hall l-iot enumerate Mr. Huygcns has computed; then ihe vdccity of 

 t}^gj^_ I Light will be to that of a cannon ball, as twenty-fi; 



The fccond fort (which is ufed to cure the bite of mad 

 dogs). grows on commons and open heaths, where the 

 Grafs is fhort, and the ground almoft bare, in moft 

 parts of England, efpecially on declivities, and on the 

 fides of pits. This ipreads on the furface of the 

 ground, and, when in perfedion, is of an Afh-co- 

 lour, but as it grows old, it alters, and becomes of a 

 dark colour. This is often carried into gardens 

 with the turf which is laid for walks and dopes, and 

 where the foil is moift and cool, it will fpread, 

 and be difHcult to deftroy, fo that it renders the 

 Grafs unfightly, but this is the only method yet 

 known to have it grow in gardens, where it is defired. 

 This is efteemed a fovereign remedy for the bite of 

 mad doo-s, and hath been for many years ufed with 

 great fuccefs. It was communicated to the Royal So- 

 ciety by Mr. George Dampier, whofe uncle had long 

 ufed this plant, to cure the bite of mad dogs on men 

 and animals, with infallible fuccels. The method of 

 taking it he has delivered as foUoweth : " Take of the 

 *' herb, and dry it. either in an oven, by the fire, or 

 *' in the fun ; theri powder it, and pafs it through a 

 *^ fine fieve ^ mix this with an equal quantity of fine 

 powdered pepper. The common dofe of this mix- 

 ture is four fcruples, which may be taken in warm 

 milk, beer, ale, or broth." . He alfo advifes, that 



cc 



«& 





V the part bitten be well waflied, as alfo the clothes of 

 . ;. the perfon who was bit, left any of the fnivel, or dri- 



. vel of the mad dog fhould remain. If the perfon bit- 

 ten be full grown, he advifes, that he be blooded be- 

 fore the medicine is taken, and to ufe the remedy as 

 foon after the bite as poflible, as alfo to repeat the 

 dofe two or three feveral mornings fafting. r v^^^,^;^ 

 LIGHT is ufed in various fcnfes: i. Sometimes it 

 - fignifies that fenfation which is occafioned in the mind 

 " i by the view of lummous bodies, ku %iV * - ; 



V a . For thofe properties in thofe bodies, whereby they 

 i.:are fitted to excite thof^ f^nfatiohs in us. Z ;;.' i - ' 



g* A certain aftion of the luminous body on the me- 

 andium between that and the eye, by the means of 

 2c;which the one is fuppofed to aft on the other, and this 



is called fecoridary Light, or derived Light, in diftinc- 

 -. tion to that of luminous bodies, which is called pri- 

 ,. mary or innate Light^. „> -. >'^ v< .• ^-' '- ; - :-, 



: As to the phsenomenon of light, philofophers have 

 i explained it feveral ways ; Ariftotle by fuppofing fome 

 bodies to be tranfparent, as air, water, ice, &c. The 

 : Cartefians have confiderably refined upon this notion 

 : of Ligh", and own, that Light, as it exifts in the lu- 

 minous body, is nothing elfe but a power or faculty 

 ? of exciting in us a very clear and vivid fenfation ; and 

 '. Father Malebranche explains the nature of Light by 

 a fuppofed analogy between it and found, the latter 

 , of which is allowed to be produced by the ihaking 

 ^ior vibration of the infenfible parts or the fonorous 



j- 





a 



•t > 



*v 



:But the greateft difcoveries into this wonderful phas- 



a nomenon have been made by Sir Ifaac Newton, that 



the primary light confifts wholly in a certain motion 



: of the particles of the lucid body, "wTiereBy'^^ey do 



Vnot propel any fiftitioOs matter fuppofed to be lodged 



An the hidden gores of tranfparent bo[die?,byj: throw 



^^j off from the luminous body certain very fojall p^ti- 



^ cTes, which are emitted every way with great force. 



;;l:Ahd the fecondary or derived Light, not in a cona- 



C- tus, but in a real motion of thefe particles receding 



;\; every way from the luminous body in /ighf^iijies, and 



i-Twith an incredible velocity. . . -. ,r v.,- / Jrj. vUt^Jl 



For it has been demonftrated by Mr. ReaumuVi frpm 



-the obfervation on the fatellites of Jupiter,' that the 



- progrefs of Light from the fun to our eg^rth- is'npt 



above ten minutes, and therefore, fyice the earth is 



at lead 1 0,000 of its own diameters diftant frg^i tljc 



fun. Light muft run 10,000 of thofe diameters in a 



minute, which is above 100,000 miles in a feconcf. 



BW' 



And if a buUet». moving, lyith the fame celerity with 



which it leaves the ipuwle^of a cannon/ requires \ 



years is to ten minutes, which is above 10,000 to i • 

 fo that the particles of Liglit move above a milfica 

 of times fwifter than a cannon ball, from which ra* 

 pidity of motion very ftrange effefts may bt pro- 

 duced; but Sir Ifaac Newton has flicv/n, paft co'j> 

 tradition, that the Light of the fun is near levcn mi- 

 nutes in its paffage to the earth, which is the fpace of 

 50,000,000, a velocity 10,000,000 times greater than 

 that wherewith a ball fiies out of the mouth of a 

 cannon. . 



Sir Ifaac Newton alfo obferves, that bodies and Light 

 ad: mutually on one another: bodies on Light, in 

 emitting, reflexihg, refrafting, and infieding it, and 

 Light on bodies, by heating therh, and puttino- their 

 parts into a vibrating motion, wherein heat principally 

 confifts i for he obferves, that all fixed bodies, when 

 heated beyond a cfertain degree, emit Light and flifj^e, 

 which fliining, &c, appears to be owing to the vi- 

 brating motion of the parts, and all bodies abounding 

 in earthy and fulphureous particles, if they be fuffi- 

 fciently agitated emit Light, which way foevcr the 

 agitation be effe^led.- 



The fame great author obferves, that there are but 



three affeftions of Light wherein the r 





differ, 



•i • 



VIZ. 



refrangibility, reflexibility, and colour j and thofe rays 



which agree in refrangibility, agree alfo in tlie other 



two, whence they may be well defined homogencal. 



-. Again, the colours exhibited by homogencal Lights 



he calls homogeneal colours, and thofe produced by 



heterogeneal Light, heterogeneal colours-, from which 



z' definitions he advances feveral propofitions: 



; -I.VfThat the fun's Light confifts of rays differing by 



*' indefinite degrees of refrangibility..; .: ' i.-^ .. 



^.j^.Tfiat jay s^_!y^hich. differ in refrangibility, whca 



parted from one another, do proportionably differ ia 



/ the colours which they exhibit, /f ;sf^;k,y>;. i>;;r 



;-3' That there are as'many fiinple and hbrnogeiwal 



colours, as there iare degrees of refrar^ibility', for to 



every degree of refrangibility belongs adifferentcolouf. , 



4. Whitenefs, in all refpefts, like that of the f«i's 

 immediate Light, and of all the ufiial ob]e£ls of our 



; fenfes, cannot be compounded of fimple colours, w'sbt- 



out an indefinite variety of them, for to fuch a c^- 



pofition there are required rays endued with all the m^ 



•i definite degrees of refrangibility, which infer as many 



• fimple colours. . •. . o.: -•. ? : ...j^Jjn^ 



5. The rays of Light do not aft one on ahotner in 

 paffing through the fame medium.^ 



6. The rays of Light do not fuffer any alteration of 

 their qualities from refraftion, nor from the adjacent 

 quiefccnt medium. r'-,n 5^; f :>nr; -.t^iita ;:';t^ 1 i:n5*l ' 



7. Th^re can be no hohfiogeneal colours produced 

 '•: out of Light by refraftion, which are hot comiiiixed 

 i in it before, fince refraftion changes not the qualifies. 

 ^; of the rays, but only feparates thofe that have divers 

 n qualities by means of their different refrangibility.? ^ 

 il9l The fun**s Light is an aggregate* of homogeneal 

 r colours, whence homogeneal colours may be calkd 



;priniijivf or original :J ;^.ei4 v^X^'^'/'^nii 

 1: Hence proceeds the whole theory of cqlours in plants 



^jand flowers. -^ Jm/ .1 '^^ ti^-*.')?* '**-^M "^"t 



■^'- ■ > 







* k -J- -» I 





> « ' 



Thofe *parts, y! g. which are th? moft refrangible, 

 conftityt^ Yjipl^t^^cplouf, thc.difnrneft and njoft; lan- 



'Va 



<L,---*-Vt 



y 



fuid of all colours. Y'r.,,.-.-. ff^fti^ f^i.^^-t o^;; . te- 

 md, on the contrary, thofe. parj^dej that ^re the 

 leaft refrangible, cpnftitute a ray or a red colour, 



: jvjiich is the brighteft and' moft vivid of all col6ub; 



; the other particle? Joeing diftipguHhed into little rays, 

 according to their refpcftive magnitudes and degnees 

 of refrangibility, excite intermediate vibrations, and 

 fo occafion the fenfations of the intermediate colours. 

 See Sir Ifaac Newton's Dodrine of Colours. 

 Perhaps thefe obfervations of Light may to fonic 

 perfons feem foreign to the fubjeft matter of this 

 book, yet, if thoroughly underftp^rnight probably ■ 

 be found very ufeful. The learned and curious en- 

 quirer into tli bufinels pf vegetation, theKev. Pr. 



^ Hales, 





^' 



