1 MH'LATORY FORCES. LIGHT. 



[rilOTOMETBY. 



to pass through tho IIO'.OM in the form of smoke, yet In- 

 coming intluonce i if th" met i! is so "i-.-at, that tho com- 

 bustible gases will not bo int1am--d. Tho same process 

 a lav r of wire gauze over the glass 



or chimney of an ordinary gas-burner : the wire will, by 

 its conducting power, so far reduce the temperature of 

 tho flame, that it will not pass through to tho apertnr s 

 of tho burner. Sir Humphry Davy took advantage of I 

 this important fact, and applied it in tho construction of 

 tho safety-lamp, which has boon of such essential si" 

 to the miner. "Tho Davy," as it is termed, is nothing 

 more than a common oil-lamp, with a cylinder or cage of 

 wire gauze surrounding tho flame 

 -. 84). This gauze permits tho 

 light, but it will not allow 

 tho flame to traverse it; and 1 

 it may be exposed to the most in- 

 flammable mixtures without danger 

 of their explosion. The minor 1. 

 when the atmosphere of the mine is 

 explosive, by the enlargement of the 

 tlamo and the burning of the fire- 

 damp within the cylinder of wire 

 gauze, and ho perceives a str 

 as it were, of the flame to pass out ; 

 but such a result is not possible, un- 

 less the cooling influence of the wire 

 is prevented by its becoming almost 

 red-hot. In this case there is dan- 

 ger ; and the fire-damp of the mine 

 may be exploded. These lamps are 

 also employed in spirit warehouses, 

 gas manufactories, in the sewers, and 

 in many other localities whore there 

 is risk from explosion. The same 

 principle was employed by Mr. 

 Hemming in the manufacture of his 

 nfuty jets for the oxy-hydrogen blowpipe. We have 

 already said that the mixed gases which are used in this 

 apparatus are exceedingly explosive; and if some con- 

 trivance wore not adopted to prevent the backing of tho 

 flame, they would assuredly take fire. In fact, in the 

 r experiments with the oxy-hydrogen blowpipe, 

 this was a result of frequent occurrence ; and Mr. Clarke 

 tells us, in his work on the subject, that he often narrowly 

 escaped being killed by the bursting of his apparatus. 

 On one occasion, when he was experimenting in the pre- 

 sence of some friends, "the reservoir for the compression 

 of tho gases, although made of thick copper, was torn in 

 pieces ; and the fragments flew with tho force of cannon- 

 shot in all directions, like tho bursting of a bomb." To 

 guard against the danger of such terrible consequences, 

 it was found necessary to protect the operator by moans 

 of a thick oaken screen. Mr. Hemming saw the disad- 

 ,f this; and adopting the facts which Sir 

 Humphry Davy had recently brought to light, he con- 

 trived a safety jet that enabled the operator to do away 

 with all the paraphernalia that had hitherto pro: 

 his life. The jet consists qf a tube of brass, about six 

 inches long and thruu-qiiarturs of an inch in diameter, 

 d full of line copper oj brass wire: the wire is cut 

 'is of six inches, then packed as close as possible 

 in tl, 1 finally wedged together by means of a 



central bar of metal. In this way a number of fine 

 t uros are left for the passage of the gas ; and tho con- 

 ducting or cooling power of the wire is so great, that the 

 flame cannot pass back to the reservoir of mixed gases. 



Anoth'-r circumstance that operates in i 

 light and heat of flame is its rarefaction. When a tlamo 

 is put under the exhausted receiver of an air-pump, it 

 first enlarges, then the light diminishes, and finall 

 extinguished. A flame of alcohol, ether, wood-spirit, 

 wax, tallow, oil, or s]>cnnaceti, is o. I when the 



n is can-ied to a fifth or sixth degree ; hyd> 



is ex t ! when it is carried to a seventh or eighth 



o; sulphur when it reaches tho eighteenth degree; 



and ph i-phorus when it arrives at the thirtieth degree. 



"Slow combustion on the surface of platinum is exlii- 



ii-gas, down to a fourfold rarefaction of 



i ; f . 



the air ; by carbonic oxide, to sixfold ; by vapour of . 

 hoi, other, or wax, to eightfold; liy nlotianl gas, t 

 or elevenfold ; by hydrogen gas, to thirteenfold ; and by- 

 vapour of sulphur, down to a twenty-fold rarefaction of 

 the air."* From this it would follow, that the light 

 emitted by any illuminating agent is not so great at the 



I a mountain as at its base, or in 

 tho tropics as at tho polos of tho earth. 

 Lastly, it should be mentioned, that 

 the supply of very cold air to a 



diminishes its light by the 

 abstraction of heat ; and hence wo may 

 improve the illuminating power of any 

 combustible substance by supplying it 

 with atmospheric air that has 

 previously warmed. Tho Rev. Mr. 

 J.owditch, of Wakeliold, has contrived 

 an apparatus whereby this may be 

 effected in tho case of common gas. 

 He places a disc of metal or a cup of 

 glass, having a hole in its centre, ou 

 to the screw which receives the burner. 

 This disc, or cup, is made the means of 

 supporting an outer glass, and thus of 

 directing the air down over the surface 

 of the hot chimney before it enters the 

 flame from below (Fig. 85). It is stated 

 that the light is increased about 25 per 

 cent, by this arrangement. 



PHOTOMETRY. 



Intensity of L'ujU and If ml. Wo know that the power 

 of light and heat diminishes as we recede from any ob 

 that produces it, and the decrease is in proportion to the 

 square of the distance : for example, if we have a certain 

 amount of light ; at a foot from its source, tho intensity 

 will be one-fourth at two feet, because the square of two 

 is four; one-ninth at three feet (3x3=9), one-sixteenth 

 at four feet (4x4=16), one twenty-fifth at live feet 

 (5x5 = 25), and so on. Upon this law, as it is termed, 

 is founded all the methods now employed for estimating 

 the relative value of different illuminating agents. The 

 practice is called Photometry; and there are various 

 modes of conducting it. Count Rumford adopted the 

 plan of adjusting the lights at such a distance from an 

 upright rod, that the shadows produced by each on a 

 white screen should be equal ; he then measured the 

 distance of each light from the rod, and, on squaring the 

 number of inches, he obtained two sets of futures, which 

 represented tho relative values of the lights : by dividing 

 the larger sum by the smaller, we obtain a product that 

 expresses the fact in simpler terms. The late Mr. Ritchie 

 contrived an apparatus that consisted of two pieces of 

 glass, placed at right angles to each other, and so dis- 

 posed, that the light from two opposite sources should 

 bo reflected upwards upon a piece of tissue-paper that 

 covered two holes in a darkened chamber. When he 

 had adjusted tho instrument so as to obtain two n 

 tions of equal intensity, ho then measured the distances, 

 and proceeded as before. Leslie and Wheatstone liave 

 also invented photometers, which are useful enough for 

 certain purposes ; but they have all given place to tho 

 very simple contrivance of 1'rofessor ISitnscn. It wi'l bo 

 noticed, that if a piece of white filtering-paper is p;.r 

 over with molted wax or spermaceti, it acquires a gi 

 appearance, and becomes translucent : if tliis bo done so 

 as to leave a spot or disc, about the size of a shilling, 

 untouched in the centre of the paper, we shall find that 

 tho apparatus v, ill have tho follow ing properties : When 

 examined by ! lit that is, with tho light on 



the same side of the paper as the observer is the disc 

 will look white, and the surrounding greasy part dark : 

 but by altering this condition of things, and lookii 

 the paper by tnilllBittod light that is, with the light 

 on the opposite side of the instrument the disc will 

 ir dark, and the surrounding greasy portion light 

 and translucent. Lastly, if two lights of equal intensity 

 Gmclln. 



