492 yldditional Notes, 



riod of the eighteenth century. But the suhject was much more 

 completely Investigated by Mr. Walker, in a paper published 

 in the Philosophical TransactiGns^ for 1195. Since that time, 

 several curious additional experiments have been made by Pro- 

 fessor LowiTZ, of Petersburgh, particularly the introduction 

 oi muriate of lime, which produces a very great degree of 

 cold when mixed with snow. The experiments of Lowitz 

 have been lately repeated and extended by Mr. Walker. 

 By means of the above-mentioned mixture, Mr. W. H. Pepys, 

 junior, of the London Philosophical Society, with the as- 

 sistance of some friends, froze, on the eighth of February, 

 1799, fifty-six pounds averdupois of mercury into a solid 

 mass. In this process, the mercury in Fahrenheit's thermo- 

 meter sunk 62 deg. below 0. a degree of cold never before 

 produced in Great-Britain. 



It is scarcely necessary to add, that these frigorific mixtures 

 produce their effect, by tlie great and sudden absorption of 

 caloric, which the materials occasion when brought together. 



Light, p. 95. 



Modern philosophers have discovered that the Influence of 

 h'ght on growing vegetables k great and important. Plants 

 may be made to vegetate tolerably well in the dark ; but in 

 that 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 considerably increased. — Thompson'^ Chemistfy. 



" It has been found by Dr. Herschell (seePhilos. Trans, 

 for 1800) that the rays of light differ in their power of illu- 

 minating objects : for if an equal portion of each of these 

 rays, one after another, be made to illuminate a minute ob- 

 ject, a printed page, 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 violet. 

 We see distinctly, at a somewhat greater distance, when the 

 object is illuminated by the indigo ray ; at a greater when by 

 the blue; at a still greater when by the deep green; and at 

 the greatest distance of all when by the lightest greeji or 

 deepest yelloiO. We must stand neaier when the object is il- 

 luminated by the orange ray ; and still nearer when by the 



