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 ;t 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. % 



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- 



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



t Dr. Young's Lectures on Nat. Phil. lect. xxxvi. vol. 1. p. 426. The equatorial helio- 

 stat appears to have been first suggested by Fahrenheit. 



X Proceed. Ajii. Phil. Soc. Sept. 17, 1841, vol. ii. p. 97. 



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



II Homberg's phosphorus is a calcium chloride prepared by melting one part of 

 sal ammoniac (amnionic chloride) with two parts of slal^ed 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. 



