A text-book on chemistryFor the use of schools and colleges . o ascertain the relativeintensities of lights, we have only to determine at what\distance they will extinguish a givenshadow. Their intensities are asthe squares of those distances. The chlorine-hydrogen photom-eter, invented by Professor J. , depends on the fact thatchlorine and hydrogen, if mixed inthe proper proportion to form hy-drochloric acid, do not unite in thedark, but, if exposed to even a fee-ble light, combine, the gases con-tracting more quickly as the lightis stronger, and turning into theacid. The contrivance
A text-book on chemistryFor the use of schools and colleges . o ascertain the relativeintensities of lights, we have only to determine at what\distance they will extinguish a givenshadow. Their intensities are asthe squares of those distances. The chlorine-hydrogen photom-eter, invented by Professor J. , depends on the fact thatchlorine and hydrogen, if mixed inthe proper proportion to form hy-drochloric acid, do not unite in thedark, but, if exposed to even a fee-ble light, combine, the gases con-tracting more quickly as the lightis stronger, and turning into theacid. The contrivance consists ofa glass tube, a b c d, Fig. 66, withthree^ platinum wares, fg /i, fusedinto it. From one end, c?, a finetube, e, projects, provided with ascale; the other end, a, is stand h supports the the instrument is to be used,it is filled with hydrochloric acid,and the wires / and g connected with a voltaic bat- What is the principle of the method of extinction ? Describe thechlorine-hydrogen photometer. How is it made ready for use ?. 92 BURNING-GLASSES. / tery, so that chlorine may be evolved from g and hydro-gen from f. As soon as the acid in the limb a b h issaturated with chlorine, the pole of the voltaic batterythat had been connected with f is dipped into the mer-cury cup of h. Chlorine and hydrogen then accumulatetogether at a 5, which part is to be covered with anopaque screen. The acid at the same time rises in thefine tube e. On taking off the screen over a b and exposing thephotometer to light, the chlorine and hydrogen at oncecommence to unite, and the acid descends in the tube amount of action can be quantitatively ascertainedby examining the scale. If exposed to sunlight, a vio-lent explosion will result, and the instrument be destroy-ed. This photometer was extensively used by ProfessorDraper in his Researches on Light, published in thePhilosophical Magazine. Light does not move from point to point instanta-neously, but at a rate whi
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