228 PROGRESS OF SCIENCE. 



the angles of crystals ; was the first to see the BLACK LINES 

 in the solar spectrum (1802), but seems to have failed to 

 perceive their significance ; he simply noted their existence. 

 This was the second step in the progress of spectrum 

 analysis. Wollaston invented the CRYOPHORUS, to freeze 

 water by means of rapid evaporation ; he invented also the 

 HYPSOMETER, to determine the height of a place by means of 

 the boiling point (see Franklin); he devised the construction 

 of the lenses of the microscope known as WOLLASTON's 

 DOUBLET. He further investigated the malleability of 

 metals, and found platinum to be the most ductile of all, 

 for he actually obtained a platinum wire O'OOOO3 of an inch 

 in diameter so fine that 1,060 yards of it weighed only 075 

 of a grain. He rendered countless services to science. 



1766 1832. Leslie (Sir John) advanced the knowledge 

 of THE PHENOMENA of reflection, emission, and absorption 

 OF HEAT science is mainly indebted to him in these 

 branches. Invented a HYGROMETER, a differential ther- 

 mometer, and a photometer (1800); also a method of 

 artificial FREEZING BY RAPID EVAPORATION; carried on 

 numerous experiments on the relation of air to heat and 

 moisture (his work on the subject a standard book); in- 

 vented a reflecting tube (bearing his name) by means of 

 which the reflecting power of substances is determined. 



1766 1844. Dalton made fine discoveries in heat: 

 determined the amount of vaporisation of liquids according 

 to heat ; discovered and tabulated the SPECIFIC HEAT OF 

 GASES. 



1773 1829. Young (Thomas), one of the most original 

 thinkers of our age, was, next to Huygens, the first to indicate 

 the right path and lay down the right principle for the 

 ELUCIDATION of the most important and complicated 

 phenomena of light, the development of which was worked 

 out by Fresnel, and led the latter to establish the UNDULA- 

 TORY THEORY OF LIGHT, although the credit of this great 

 discovery is often but erroneously assigned to Young alone. 

 Young, as early as 1801, made experiments upon the "inter- 

 ference of light " and to him is due this denomination of the 

 phenomenon ; but Fresnel, once in possession of the con- 



