C II KM IS TRY 



Eiemcuti different distance it is bcnt./rom it. In the first case, 

 _.*? the ray is said to be infected, in the second to be de- 



Light'com- When a ray of light is made to pass through a 

 I of triangular glass prism, and received upon a sheet of 

 1 r ' vu paper, the image or sjtectrum, as it is called, instead 

 of being round, is oblong. This spectrum exhibits 

 seven different colours, in the following order, be- 

 ginning with the lowest ; red, orange, yellmv, green, 

 blue, indigo, violet. In this case the refraction of the 

 ray is increased by the figure of the prism j and if it be 

 heterogeneous, and consist of rays differing in refran- 

 gibility, they will separate from each other, the most 

 refrangible going to the top of the spectrum, the 

 least refrangible to the bottom, and the others in their 

 order. This is the case. Light consists of seven 

 different rays, distinguished by seven different colours. 

 The red is the least refrangible, and the violet the 

 most. The refrangibility of the rest is in the order 

 of their names. 



The rays of light differ in their power of illumina- 

 ting objects. The lightest green or deepest yellow 

 gives the most light, and the light diminishes as we 

 approach either extremity of the spectrum. The 

 violet has the least illuminating power. 



Exists in Light is capable of entering into bodies and re- 



bodies. maining in them, and of afterwards being extricated 

 by various means. Such bodies are said to phosplto- 

 resce. Almost all bodies possess this property to a 

 certain extent. If they be exposed to the sun, and 

 suddenly carried to the dark, they are luminous for 

 some time, but in general for a very short period. 

 Some bodies seem to contain light as a constituent, 

 from which it may be extricated by various means. 

 Thus fluor spar, and various other minerals, become 

 luminous when heated. Herring, other fish, meat, 

 and wood, often become luminous just before they 

 begin to putrefy, and often continue luminous for a 

 considerable time. 



Produces Light produces considerable changes upon certain 

 chaugts on bodies. The green colour of plants is owing to it; 

 hodie *' for when they vegetate in the dark, they are white. 

 Nitric acid is decomposed by exposure to the light, 

 and oxygen gas emitted. The oxide of silver, and 

 perhaps also of gold, is reduced by exposure to light. 

 Till lately, it was supposed that these changes were 

 produced by the colorific rays of light. But it has 

 been recently ascertained, that muriate of silver is 

 blackened most rapidly when placed beyond the vio- 

 let ray, and entirely out of the prismatic spectrum. 

 Hence it follows, that the change is produced, not 

 merely by the colorific rays, but by rays which are 

 incapable of rendering objects visible, or of producing 

 any sensible heat. Thus we learn that the solar light 

 contains at least two distinct sets of rays, one *et 

 which renders bodies visible, and another which 

 blackens muriate of silver, and reduces metallic 

 oxides. This second set may be called deoxidizing 

 rays, till some better name is thought of. Tin y are 

 obviously more refrangible than the colorific r<ys. 



Such are the properties of light. They are sulTi 

 cient to induce us to believe that it is a body; but it 

 possesses three peculiarities, by which it is distin- 

 guished from all the substance* hitherto described. 

 It has the power of exciting in us the sensation of 



; it always moves with I prodigious velocity, Element* 

 aiul the particles of it are never found cohering to- 

 gether in masses. This last property cannot well ^__ 

 be accounted for, unless we suppose that its particle* 

 repel each other. 



The sources of light are, the tun and stars, com- Source*. 

 bttsltOH, heat, and percussion, 



The light emitted by the sun is familiarly known 

 by the name of the light of dm/. In all cases of rapid 

 combustion li^ht is emitted : but different substances 

 vary very much in the quantity of light which they 

 give out while burning. All substances, except gases, 

 become luminous when heated to a certain temperature 

 (about 700). They are then said to be red hot. 

 AVIien hard substances, as two quartz stones, flmt 

 and steel, are struck against eacli other, luminous 

 sparks are emitted. This is sometimes, (as in the 

 case of flint and steel) owing to the particles given 

 off catching fire ; but in other cases, trie appearance 

 of the spark has not been accounted for. 



CHAP. II. 

 Of Caloric. 



THE meaning of the word heat is so well under- Caloric, 

 stood, that any attempt to define it is unnecessary. 

 When we say tlint a person ./ir/.v /tent, that a stone it 

 hot, the expressions are understood readily. Yet in 

 each of these propositions, the word hrut has a dis- 

 tinct meaning. In the first it signifies the sensation 

 of heat ; in the second, the cause of that sensation. 

 to avoid the supposed ambiguity of these two mean- 

 ings to one word, the term caloric was invented to 

 signify the cause of heat. When I put my hand on 

 a hot stone, I experience a certain sensation, which I 

 call the sensation (>/' Itnit ; the ctuitf of this sensation 

 is caloric. The phenomena of heat, which are of the 

 utmost importance in chemistry, will be treated of in 

 the following sections. 



SECT. I. Of the Nature of Caloric. 



Two opinions respecting the nature of caliric have Nature of 

 divided philosophers. According to some, like gra- caloiic. 

 vity, it is merely a property of matter, while others 

 consider it as a peculiar substance. The latter opi- 

 nion was first broached by the chemists, and is at 

 .present acceded to hy almost the wliol' body of phi- 

 jiht-rs. A recent di>covi ry of Dr Herchel has 

 renden d this opinion, if possible, still more plausible 

 than before. 



Dr Herschel, while employed in examining the Rays of 

 sun by means of telescopes, thought of examining the 

 heating power of the different r.iys separated by the 

 prism. He found that the most refrangible rays 

 have the Ira^t heating power, and that the heating 

 power gradually increases as the refrangibility dimi- 

 nishes. The violet ray of course has die least, and 

 the rcil ray the greatest heating power. It struck 

 Dr Herscriel as remarkable, that the illuminating 

 power and heating power follow diffi tvnt laws. The 

 illuminating power i.i greatest in the middle of the 

 spectrum, but the healing power is greatest at the 



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