Intensity of Rays of High Ref Tangibility . 283 



ratus on the beam of a balance in such a manner that the 

 flow of acid from one arm to the other produced a movement 

 of the beam, which was communicated by means of a lever to 

 a pen and was recorded on a rotating drum. 



As has already been stated, a differential actinometer, con- 

 taining air in one bnlb and chlorine in the other, gradually 

 becomes inaccurate owing to the diffusion of chlorine into the 

 air-bulb. This difficulty is avoided in the apparatus shown 

 in fig. 3, Plate III., which consists of two differential arrange- 

 ments A B C D and E F Gr H, suspended on the beam of the 

 balance I ; one is completely filled with dry chlorine, the tubes 

 B and D being half filled with strong sulphuric acid saturated 

 with the gas. The second contains dry air with sulphuric 

 acid in F and H. One of the chlorine-bulbs A and one of 

 the air-bulbs E are exposed to light, whilst the other chlorine- 

 bulb C and air-bulb G are protected from the light by a covering 

 of tin-foil, and hang in the box J, in which the two bulbs 

 €an swing freely. It will be seen that when A and E are 

 exposed to the heating-effects of sunlight, the expansion of 

 the gas in A causes acid to flow from B to D, whilst a 

 corresponding expansion in E causes an equal weight of acid 

 to flow from H to F. But the chlorine in A undergoes a 

 further expansion, due to the actinic rays, causing an 

 additional weight of acid to pass from B to D. A movement 

 of the beam is thus produced which is communicated to the 

 lever M, and is registered by means of a pen on a strip of 

 curve-paper rotating on the drum N. The capacity of the 

 bulbs A and E in this case is 273 cub. centim., that of Gr- 

 and C 319 cub. centim.; the tubes B and F and D and Hare 

 15 centim. long and 1*25 centim. in diameter, the distance 

 between the two sets of tubes being 35 centim. It is found 

 desirable that the movement of the bulbs and beam should 

 be as small as possible, and that the effect should be magnified 

 by increasing the length of the lever. After the apparatus 

 is blown together, care must be taken to remove all moisture 

 by drawing dry air through the bulbs and tubes. The 

 apparatus is exposed out of doors in a wooden box Gr, fig. 4, 

 Plate III., the bulbs A and B passing through a hole in the 

 lid; these are protected by a glass shade, C, 45 centim. high 

 and 20 centim. in diameter ; this is mounted in a block of 

 wood, D, so that when the box is in its normal position, the 

 axis of the cylindrical shade points to the pole star. The two 

 bulbs occupy a position equidistant from the sides of the 

 shade and about midway between the top and bottom. The 

 tube E serves as a ventilator and should be at least 2 "5 

 centim. in diameter ; the cap F protects the opening from 



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