5io HISTORY OF SCIENCE. 



of transmitting light. Thus clear quartz is as transparent as the finest 

 glass, while "smoky" quartz is dark brown and almost opaque; never- 

 theless, these substances are alike in their power of transmitting heat- 

 rays, except a slight difference in the case of rays from the lamp, of 

 which the smoky quartz transmits 37 per cent, while the clear quartz 

 transmits 38 per cent. 



One of the facts just mentioned may find a familiar experimental 

 demonstration in the drawing-room fire-screens, made of plate-glass, 

 which are now not uncommon. Such a screen permits the luminous 

 radiations to pass, but cuts off the hot non-luminous rays, of which 

 most of the calorific radiation consists. 



The diathermancy of gases was the subject of a very elaborate in- 

 vestigation in the hands of one of the best known of our English 

 physicists, DR. TYNDALL. The eminence of this gentleman, not only 

 as an investigator, but as one of the ablest and most eloquent of all 

 our expositors of science, will render interesting the following brief 

 notice of his career, extracted from " Men of the Time." 



"John Tyndall, LL.D., F.R.S., was born about 1820, in the village 

 of Leighlin Bridge, near Carlow, in Ireland. His parents were in very 

 humble circumstances, but they gave him the best education in their 

 power, and sent him to a school where he acquired a sound knowledge 

 of mathematics. At the age of nineteen he joined in the capacity of 

 "civil assistant" a division of the Ordnance Survey which was sta- 

 tioned in his native town. In 1844 he was engaged by a firm in 

 Manchester, and for about three years he was employed in engineering 

 operations in connection with railways. In 1847 he accepted an ap- 

 pointment as teacher in Queenwood College, in Hampshire, a new 

 institution, devoted partly to a junior school and partly to the pre- 

 liminary technical education of agriculturists and engineers. Here he 

 became acquainted with Mr. (now Dr.) Frankland, who was resident 

 chemist to the college, and here he commenced those original inves- 

 tigations which have placed him in the foremost rank among the ex- 

 plorers of science. In 1848 the two friends quitted England together 

 and repaired to the University of Marburg, in Hesse-Cassel, where 

 they studied under Bunsen and other eminent professors. Afterwards 

 Mr. Tyndall prosecuted his researches in the laboratory of Magnus, 

 at Berlin. He conducted investigations on the phenomena of dia- 

 magnetism, and on the polarity of the dia-magnetic force, including 

 researches on the magneto-optic properties of crystals and the relation 

 of magnetism and dia-magnetism to molecular arrangement. He has 

 recently published a volume on these subjects. In 1853, having been 

 previously elected a Fellow of the Royal Society, he was chosen Pro- 

 fessor of Natural Philosophy in the Royal Institution of Great Britain, 

 and succeeded the celebrated Faraday as Superintendent. The pub- 

 lication of an essay on the cleavage of slate rocks was the proximate 

 cause of his joining his friend Professor Huxley in a visit to the glaciers 



