DISCOURSE OF W. B. TAYLOR. 267 



useless, since it tends to satisfy the mind with the semblance of truth, 

 and thus to render truth itself less an object of desire." * 



Light and Heat. Henry also made important investigations on 

 some peculiar phenomena connected with light and heat. For the 

 purpose of experimenting on sun-light he devised in 1840, a very 

 simple form of heliostat, based on the suggestion of Dr. Young, 

 whereby the solar ray was received into an upper room in a direc- 

 tion parallel to the earth's axis, by means of a simple equatorial 

 movement of the reflector ;f which was effected by the aid of a 

 common cheap pocket watch placed on a small hinged board set by 

 a screw to the angle of latitude. The mirror mounted on a swivel 

 and properly balanced, presented no sensible resistance to the run- 

 ning of the watch, which was arranged for the 24-hour rotation by 

 a watchmaker of Princeton. The whole cost of the completed in- 

 strument (including the time-movement) was but sixteen dollars. 

 If any particular direction of the ray was required, it was only 

 necessary to place a stationary mirror in the fixed path of the ray, 

 adjusted to the desired angle. J 



In 1841, on repeating experiments of Becquerel and Biot on 

 "Phosphorescence," he discovered some new characteristics in the 

 emanation (particularly when excited by electrical light) which had 

 not before been observed. These were more fully detailed in a 

 communication made to the American Philosophical Society, in 

 1843, "On Phosphorogenic Emanation." This phenomenon had 

 been first observed in the diamond, when taken into a dark room 

 immediately after exposure to direct sunlight, or to a vivid electric 

 spark ; and was afterward observed in several other substances, 

 notably in the chloride of calcium "Homberg's phosphorus." || 

 It had also been shown by Becquerel that while this phosphores- 



* Proceed. Am. Phil. Soc. Nov. 6, 1846, vol. iv. pp. 287-290. 



t Dr. Young's Lectures on Nat. Phil. lect. xxxvi. vol. i. p. 426. The equatorial helio- 

 stat appears to have been first suggested by FAHRENHEIT. 



I Proceed. Am. Phil. Soc. Sept. 17, 1841, vol. ii. p. 97. 



I Proceed. Am. Phil. Soc. April 16, 1841, vol. ii. p. 46. 



|| HOMBERG'S phosphorus is a calcium chloride prepared by melting one part of 

 sal ammoniac (ammonic chloride) with two parts of slaked lime. CANTON'S phos- 

 phorus is a calcium sulphide formed by a mixture of three parts of sifted and cal- 

 cined oyster shells, and one part of flowers of sulphur, exposed for an hour to a strong 

 heat. 



