THE ASTRONOMICAL VIEW OF NATUEE. 315 



stars were speeding through the universe; without them 

 these objects of nature would probably never have been 

 seen, and if seen, they would not have been recognised. 

 Similar, and still more intricate, reasonings permitted 

 Mendeleeff 1 to arrange in geometrical order the several 

 elements or simple substances out of which matter is 

 compounded, and to point to the vacant places on the 

 chart, some of which have since been filled up by new 

 discoveries. Thus it has also been shown that the ranges 

 of temperature cannot be extended indefinitely in both 

 directions viz., those of heat and cold but that the 

 latter possesses a zero point, representing the complete 

 absence of motion. 2 



1 The periodic arrangement of 

 the elements, according to which, 

 with increasing atomic or combining 

 numbers, the same properties such 

 as density, fusibility, optical and 

 electric qualities, and formation of 

 oxides, &c. recur in periods which 

 are at least approximately fixed, so 

 that they can be represented by 

 curves, dates from the year 1869, 

 when D. Mendeleeff and Lothar 

 Meyer published almost simultane- 

 ously their classification of the ele- 

 ments. Newlands seems to have 

 indicated some of these facts as 

 early as 1864. Mendeleeff pre- 

 dicted the properties of a missing 

 element, found to be those of scan- 

 dium, which Nilson discovered ten 

 years later. The same applies to 

 the two other elements which were 

 subsequently discovered by Lecocq 

 de Boisbaudran (1878, gallium) and 

 Winkler (1886, germanium), and in 

 1894 the newly discovered element 

 argon was found to fill a vacant 

 place in the plan. 



2 The zero point of temperature 

 was originally a purely mathemati- 

 cal quantity suggested by the for- 



mula which gives the expansion of 

 air in the air thermometer as de- 

 pendent on the temperature. The 

 ideal, not realisable, temperature 

 at which, according to the for- 

 mula, the volume of air would be 

 nothing, was fixed by calculation at 

 459-13 Fahr. or 272'85 Centi- 

 grade. The real physical, not mere- 

 ly mathematical, meaning of the 

 absolute scale of temperature with 

 its zero point was only revealed 

 when, through Carnot and Thom- 

 son, it was established that every 

 degree of temperature has an assign- 

 able value for doing work, and when 

 a scale of thermometry was sug- 

 gested by Thomson (1848) in which 

 every one degree had the same 

 dynamical value, 100 in it cor- 

 responding to the 100 Centigrade 

 in the air thermometer. It was 

 then found that the two scales 

 that of the air thermometer and 

 that measuring the dynamical value 

 of temperature agreed almost ex- 

 actly. The number 273 Cent, thus 

 acquired a physical meaning (see 

 Clerk Maxwell, 'Heat,' 8th ed., 

 pp. 49, 159, and 215). Another 



