360 PHYSICS. 



series of spectra by which the substances used can be identified, thus 

 forming a true acoustic spectrum analysis. {Am. J. Sci., June, 1881^ 

 III, xxi, 463; G. B., xcii, 1206; Phil 3Iag., June, 1881, V, xi, 510; Xa- 

 ture, May, 1881, xxiv, 42.) 



In a third paper, Graham Bell has described a modification of Wheat- 

 fecone's microphone, and pointed out its applicability to radiophonic re- 

 searches. Preece's failure to detect the vibration of the diaphragm was 

 due to the fact that he used a Hughes form of the instrument, in which 

 the points of support are too far from the center where the maximum 

 vibration exists. In 1827, Wheatstone invented a microphone, consist- 

 ing of a metallic diaphragm, to the middle of which a stiff wire was 

 rigidly attached. By inclosing this ina case somewhat like that of a 

 telephone, the wire projecting through the end of the handle and a 

 tube for hearing being fitted to the opposite end, the surface of the 

 radiophonic diaphragm may be explored. When it rested on the center 

 of this diaphragm a clear musical note was heard, showing that the 

 diaphragm itself vibrated. [Am. J. Set., August, 1881, III, xxii, 87.) 



Ayrton and Perry, observing the facility with which the invisible 

 rays which affected the selenium in Bell's photophone passed through 

 ebonite, concluded that these rays would be refracted by an ebonite 

 prism. This conjecture they were able to confirm exj)erimentally. 

 Moreover, by suitably arranging the apparatus, the prism having a re- 

 fracting angle of 27°.6, they succeeded in measuring the index of re- 

 fraction for these rays, which they found to be 1.7. This result accords 

 with that obtained by Jellett from the polarizing angle, 1.611, and with 

 that obtained when the light is very intense, so that the red rays can 

 be faintly seen, 1.66. {Phil. Mag., Sept., 1881, V, xii, 196; Nature, 1881, 

 xxiii, 519; J. Phys., November, 1881, x, 507.) 



4. Specific heat. 



Mallet has described a simple form of calorimeter for determining 

 the specific heat of solids and liquids with small quantities of material. 

 It consists of a cylinder of vulcanite 105 millimeters long and 64 milli- 

 meters inside diameter, 1.5 millimeters thick, closed at the ends by 

 round plates of the same material screwed on. Within this is an inner 

 cylinder, also of vulcanite, 22 millimeters in interior diameter, passing 

 closely through a hole in one of the end caps and screwing into the 

 other. The space between the two is filled with vulcanite shavings. 

 Both ends of the inner tube are closed by corks, through one of which 

 passes the stem of a mercurial thermometer, graduated to tenths. A 

 diaphragm with a hole 7 millimeters in diameter in its center is fixed 

 in the inner tube 47 millimeters from one end, and carries three plati- 

 num wires so bent as to hold a small ijlatinum cylinder firmly. This 

 cylinder, intended to contain the substance for experiment, is 28.5 mil- 

 limeters long by 12.5 millimeters diameter, weighing with its cover 

 grams. The outer cylinder is mounted on trunnions and supported ou 



