XXI.] THE SIMPLEST ELEMENTS APPEAR FIRST. 165 



With regard to the general question of inorganic evolution, the 

 first idea was thrown out in the year 1815 by Prout, who, in conse- 

 quence of the low atomic weight of hydrogen, suggested that that 

 substance was really the primary element, and that all the others, 

 defined by their different atomic weights, were aggregations of hydro- 

 gen, the complexity of the aggregation being determined by the 

 atomic weight ; that is to say, the element with an atomic weight of 

 20 contained 20 hydrogen units ; with an atomic weight of 40 it 

 contained 40, and so on. The reply to that was that very minute 

 work showed that the chemical elements, when they were properly 

 purified and examined with the greatest care, did not give exactly 

 whole numbers representing their atomic weights. They were so and 

 so plus a decimal, which might be very near the zero point, or half-way 

 between, and that was supposed to be a crushing answer to Prout's 

 view. The next view, which included the same idea that is to say, a 

 physical connection between these different things as opposed to the 

 view that they were manufactured articles, special creations, each 

 without any relation whatever to the other, was suggested by Dobe- 

 reiner in 1817, and the idea was expanded by Pettenkofer in 1850. 

 Both pointed out that there were groups of three elements, such as 

 lithium, sodium, and potassium, numerically connected; that is, their 

 atomic weights being 7, 23, and 39, the central atomic weight was 

 exactly the mean of the other two, 7 + 39 = 46, divided by 2, we 

 get 23. Another way, however, of showing that is that 7 + 16 =23, 

 and 23 + 16 = 39 ; the latter method suggests a possible addition of 

 something with an atomic weight of 16. 



In 1862 de Chancourtois came to the conclusion that the relations 

 between the properties of the various chemical elements were really 

 simple geometrical relations. It is not till 1864 that we come to the 

 so-called " periodic law," which was first suggested by Newlands, and 

 elaborated by Mendeleef in 1869. According to this law, the chemical 

 and physical properties of the elements are periodic functions of theii 

 atomic weights. Lothar Meyer afterwards went into this matter, and 

 obtained some very interesting results from the point of view of 

 atomic volumes. He showed that if we plot the atomic volumes of 

 the different elements, arranged according to their atomic weights from 

 left to right, there is a certain periodicity in the apices of the curve 

 indicating the highest atomic volumes. 



So far there was no reference to the action of temperature in rela- 

 tion to this, but in 1873 I suggested that we must have a fall of tem- 

 perature in stars, and that the greater complexity in the spectra of 

 certain stars was probably due to this fall of temperature. This idea 

 was ultimately utilised by Sir William Crookes in an interesting varia- 



