Habitudes of Light.— Chemical Nomtnelalure. jfyf 



of air between tlicm, appear to have a different caufe, as may be without much reafoning 

 gathered from the curious experiments of the Abbe Mazeas*, and even from one or two 

 of Sir Ifaac's own, in which he fuppofes fome medium more fubtle than air to be between 

 the glaflcsf- But at prefent I forbear entering into the fubjeft any farther. This paper 

 has already been extended to a greater length than was at firtt intended. And I haften to 

 conclude by a fhort fummary of propofitions, containing the principal things which have 

 been demonftrated in the courfe of it. 



Prop. I. The angles of inflexion and deflexion are equal at equal incidences. 

 Prop. 11. The fine of inflexion is to that of incidence in a given ratio (wliich il deter- 

 mined in the paper). 



Prop. III. The fun's light confifts of parts whicTi differ in degree of inflexibility and de- 

 flexibility, thofe which are moft refrangible being leafl flexible. 



Prop. IV. The flexibilities of the rays are inverfely as their refrangibilities ; and the fpec- 

 trum by flexion is divided by the harmonical ratio, like the fpeftrum by refradion. 



Prop. V. The angle of reflexion is not equal to that of incidence, except in particular 

 (though common) combinations of circumltances, and in the mean rays of the fpettrum. 



Prop. VI. The rays which are moft refrangible are leall reflexible, or make the lead angle 

 of reflexion. 



Prtp. VII. The reflexibilities of the difl^erent rays are inverfely as their refrangibilities, and 

 the fpe£trum by reflexion is divided in the harmonical ratio, like that by refraflion. 



Prop. VIII. The fines of reflexion of the difl"crent rays are in given ratios to ihofe of 

 incidence (which are determined in the paper). 



Prop. IX. The ratio of the fizesof the difl^erent parts of light are found. 

 Prop. X. The colours of natural bodies are found to depend on the different reflexibilltieg 

 of the rays, and fometimes on their flexibilities. 



Prop. XI. The rays of light are refle£lsd, refrained, inflefted, and defle£led, by one and 

 the fame power varioufly exerted in different circumflaiices. 



SIR, 



III. 



On certain Points of Nomendattin: By a Ccrrefpotideiit. 

 To Air. NICIIOI.S.ON, 



XT appears to me, that the new name tartnrin, propofed to dcfignate the fubflance com- 

 monly called vegetable alkali, is more objedionable than fome of tlie number of names 

 which are in ufe. 



1. Bccaufe the word tartar is familiarly known to always denote a fait of acid properties 

 which confifts of nearly three parts of tartareous acid, and one of vegetable alkali. 



2. The word tartanous acid, which is already employed to denote the acid contained in 

 tartar (and for which fubftance there is no other name), fo nearly refemblcs the word tartarin 

 as to make this name an improper one for the alkali of tartar and of vegetables in general. 



i* Mem, dc I'Acadcmie pcur TAnncc 1738, f Oinics, Book ii. Vau I. Obf, x, and xi. 



3. But 



