SciEXTiFic Lectures. 137 



the porous cup containing the slab of carbon, we place a solution of 

 bicliroraate of potash, and upon the outside, in the outer jar, we pom- 

 diluted sulphuric acid. We have here a battery of 250 of these jars, 

 which the gentlemen who have kindly assisted me in preparing for 

 this lecture, put together between 10 and 2 o'clock last night. By 

 bringing together the copper wires so as to complete the circuit, we 

 obtain what is called the electric light. It is a small point, but 

 intensely hot ; and the light increases in brilliancy as the metal is 

 consumed and ih.e distance between the points is increased. Substi- 

 tuting for this point of copper, one of a mixture of copper and zinc, 

 we have a light giving us white fumes from the zinc, and a greenish 

 tinge from the copper. 



jSText I take a point of iron, wliicli produces a brilliant light; and, 

 as you see, as the iron is consumed the arc increases ulitil at last the 

 distance between the points is so great that the electricity cannot 

 bridge the space. [The dazzling light, when that distance was 

 reached, suddenly ceased.] JS^ow, employing a pencil of carbon, we 

 shall have a still more intense light, this being that which is known 

 to us now as the galvanic light. By separating the poles we break 

 the current of electricity, and thus the light is lost. In order that 

 you may fairly judge of these different degrees of light, let me bring 

 before vou the means by which we measure the intensitv of lio-lit — 

 the instrument which is the subject of our lecture — the photometer, 

 or light-measurer. We may compare lights in two or three different 

 wa,ys. We have several forms of photometers. Thus we sometimes 

 measure the light of the sun by the thermometer, by measuring the 

 amount of heat set free from that magnificent orb. We learn from 

 the latest investigations the fact that our earth receives only a very 

 small part of the light and warmth emanating from the sun, which 

 is itself so large that were it hollow, and the earth placed in its center, 

 there would be almost room enough for anotlier moon outside our 

 satellite, within the surface. The sun sets free as much heat as would 

 be produced by burning no less than seventeen miles in thickness of 

 coal covering its whole surface, within a single year. But our earth 

 receives a very small portion of this lieat and light. Conceive a great 

 concave sphere, with the sun in its center, and the orbit of our earth 

 in its circumference, and you will see what an infinitesimal portion 

 of the surface of that sphere would be occupied by our earth. 



We thus learn that our globe receives but one 2,300,000,000th part 

 of the heat set free from the sun. Another method is by measuring 



