420 Additional Notes, 



tant. Plants may be made to vegetate tolerably well in the dark j 

 but in this case their colour is always white, they have scarcely 

 any taste, and contain but a very small portion of combustible 

 matter. In a very short time, however, after their exposure to 

 light, their colour becomes green, their taste is rendered much 

 more intense, and the quantity of combustible matter is consi- 

 derably increased. — ^Thompson's Chemistry. 



'' It has been found by Dr. Herschell (see Vhilos, Trans, for 

 1 800) that the rays of light differ in their power of illuminating 

 objects : for if an equal portion of each of these rays, one after 

 another, be made to illuminate a minute object, a printed pago 

 for instance, it will not be seen distinctly at the same distance 

 •when illuminated by each. We must stand nearest the object 

 when it is illuminated by the viokt. We see distinctly, at a 

 somewhat greater distance, when the object is illuminated by the 

 indigo ray; at a greater when by the bhw ; at a still greater 

 when by the deep green; and at the greatest distance of all 

 when by the lightest green or deepest yellow. We must stand 

 nearer when tlie object is illuminated by the orange ray ; and 

 still nearer when by the red. Thus it appears that the rays to- 

 wards the middle of the spectrum possess the greatest illumi- 

 nating power, andtliose at tlie extremity the least j and that the 

 illuminating power of tlie rays gradually :. diminishes from the 

 middle of the spectrum towards its extremities." — Ihid. 



Note (W), p. 131. — In Fahrenheit's thermometer the freezing 

 point is fixed at 32 deg. and the boiling point at 212 deg. In 

 Reaumur's, or rather de Luc's, the freezing point is O, and the 

 boiling point 80 deg. In Delisle's, the usual order of gradua- 

 tion is inverted, tlie freezing point being 150 deg. and the boil- 

 ing point 0. And, finally, in the thermometer of Celsius, tlie 

 point of freezing is marked 0, and that of boiling 100 deg. To 

 reduce the degrees of Reaumur to those of Fahrenheit, tlie fol- 

 lowing formula may be employed :-^^ — i--f-32=F. To reduce the 



4 

 degrees of Celsius to those of Fahrenheit, the following is suffi- 

 cient : -1?_? -f 32=F. To reduce the degrees of Delisle, 



5 

 \inder the boiling point, to those of Fahrenheit, say, 212 ~ 



«=F, To reduce those above the boiling point to Fahrenheit, 



5 



say, 2!2 + ^j^=:;F. 

 5 



