280 Prof. Tyndall on the Polarization of Heat. 



unchanged in weight. At 212° F. tabular selenite lost in about 

 an hour 2*41 per cent. ; fibrous gypsum lost nothing the first 

 half hour, and only 0*55 per cent, during about an hour. On 

 continued heating at this temperature loss increased rapidly in 

 both cases and became pretty uniform ; at least the tabular lost 

 in about six hours 14*1 per cent., while the fibrous lost from 

 12 to 15 per cent, in different specimens. On heating at a 

 higher temperature, the total loss was 20*77 by the tabular and 

 20*82 by one specimen of fibrous gypsum, both numbers which 

 agree with 20*88, the percentage of water in pure gypsum. 



Regaining of Water by Dried Gypsum. — Tabular gypsum 

 which had lost 20*77 per cent, water regained in about two 

 days, when left in a room, 5*47 per cent, of its weight ; fibrous, 

 which had lost 20*71 per cent., regained in about thirty-six 

 hours only 0*02 per cent.; fibrous which had lost 20*21 per 

 cent, regained in two days 3*97 per cent. ; and exposure for 

 three hours at an open window on a damp October day gave 

 scarcely any perceptible increase in either of these two cases. 

 Fibrous gypsum which had lost 5*37 per cent, at 212° F. re- 

 gained only *07 per cent, of its weight during exposure for some 

 days in a room, and for three hours of the time to the damp 

 open air. Hence it appears that tabular selenite both loses 

 water more readily than fibrous gypsum, and regains more 

 quickly that which has been expelled by moderate heat. What 

 light these and other results, obtained by experiments of an allied 

 nature, may throw on the mutual relations of gypsum and an- 

 hydrite, and the supposed derivation of the former from the 

 latter, before touched upon*, remains to be seen. 



Windsor, Nova Scotia. 

 February 24, 18/0. 



XXXVI. On the Polarization of Heat. 

 By Professor Tyndall, F.R.S., fyc.-f 



IN the Philosophical MagazineYor November 1835 the late Prin- 

 cipal Forbes gave an account of the experiments by which he 

 demonstrated the polarization of non-luminous heat. He first ope- 

 rated with tourmalines, and afterwards, by a happy inspiration, 

 devised piles of mica plates, which from their greater power of 

 transmission enabled him more readily and conclusively to esta- 

 blish the fact of polarization. The subject was subsequently fol- 

 lowed up by Melloni and other philosophers. With great saga- 

 city Melloni turned to account his own discovery, that the ob- 



* Contributions, III., Phil. Mag. January 1868. 

 t Communicated bv the Author. 



