New Procefs for making Vegetable Lahet. l^S 



tlie fpot& and fringes evidently were diftended in breadth. I have endeavoured to exhibit the 



fun's image, as mottled with fringes or rings and fpots, in fig. 5. 



ObfervaUon 4. — I placed the fpeculum behind a fcreen with a hole in it, through which 



were let pafs the homogeneal rays of the fun, feparatcd by refradion through a prifm ; thil 



being turned on its axis, the rays which fell on the fpeculum were changed ; the fringes 



were now of that colour whofe rays fell, and when the rays fliifted, the fringes contrafled 



or dilated, being broadcft in the mofl flexible rays, and confequently in thofe whofe flexity 



is greateft. 



[Ti le csnitnued.'] 



III. 



Enquiries refpeSltng the Colouring Matter of Vegetables, and ihe Atl'ion of Metallic Suljlances and 

 their Oxides upon them; together with a New Prccefs for obtaining Lakes of the moji in- 

 ien/e and falid Colours. Read to the National Injlitute (of France) 1 5 Vendemiairey in the 

 YearVL By the C.GuTTON*. ' 



J_^INN JLUS, the great naturalift of the north, had affirmed that the red colours of ve- 

 getables announce the prefence of an acid. It was long ago obferved, that the juice of the 

 violet acquires a beautiful blue {hade in veffels of tin, the ufe of which metal was recom- 

 mended in difpenfatorles for the preparation of violet fyrup ; and the original colour of fuch 

 fyrups as had been changed by keeping, was reftored by long digeftion in tin. Little 

 attention however was paid to the caufe of thefe phenomena ; and our aflociate, BerthoUet, 

 in his Elements of the Art of Dyeing, had pointed it out no otherwife than by conjecture, 

 when he fuppofed an acid to have combined with the oxide formed at the furface of the 

 tin. 



Such was the ftate of our knowledge on this fubjeCl, when, from the ftriking difference of 

 colour of two preparations of the fame fruit, I undertook to examine the circumftances in 

 which thefe changes take place. 



1 fupprefs the detail of experiments to which I fubjeded almoft all the acid coloured fruits 

 in fuccefTion, fuch as the ftrawberry, the goofeberry, the plum, as well as the petals 

 of flowers, turnfol, fernambouc, turmeric, &c. by treating them comparatively in veflels 

 of glafs, of porcelain, of metal, and metallic alloys, or by keeping them in digeftion on 

 plates of metal perfedly cleaned, or upon metallic oxides. I fhall confine myfelf at prefent 

 to fuch refults as may improve the theory of vegetable colours, or afford fome ufeful appli- 

 cations to the procefles of the arts. 



Thefe experiments prove that the red colour of fruits is manifeftly owing to the real aftion 

 of their peculiar acid upon their colouring matter. 



That tin, when it brightens or reftores the colour of violets, docs nothing more than re- 

 fume, by fuperior affinity, the acid which had caufed it to turn red. 



That tin or its oxide is not, as has hitherto been thought, the only metal which exercifes 



* Tranflated from La Decade phJIof. litt. et politique, No, II. An VI. (J»n. 1 798.) : 



X 2 thfs 



