Chemical Substances at different Stellar Temperatures. 401 



are justified in assuming that the substances in (2) would be visible 

 in the stellar reversing layers if they were there. 



3. In the stars of higher temperatures we deal generally with gases. 

 Below the stages represented by /? Orionis and 7 Lyrae we deal 

 with proto-metals and metals, hydrogen being the only exception. 



4. The proto-metals make their appearance at about the same heat- 

 level at which the gases (with carbon), always excepting hydrogen, 

 begin to die out. 



This is the second important differentiation. It is interesting to 

 notice the distinct difference of behaviour of carbon and silicium in 

 the descending series; the former goes through the same stages as 

 oxygen and nitrogen, the latter behaves like the proto-metals. 



5. With the exception of iron the metals, as contradistinguished 

 from the proto-metals, only make their appearance in stars at and 

 below the heat-level of Sirius. 



This is the third important differentiation. It is accompanied with 

 a notable diminution of hydrogen and proto-magnesium, and with an 

 increase of proto-calcium : indeed the latter seems generally to vary 

 inversely with the hydrogen. 



In all these changes we seem to be brought into presence of succes- 

 sive polymerisations due to reduction of temperature. Of the origin 

 of proto-magnesium and proto-calcium the stars as yet tell us nothing, 

 but it is difficult to believe that the earliest forms of the other metals 

 are not built up of some of the constituents of the heat ranges repre- 

 sented by those between y Argus and a Crucis. 



The question arises whether the order of visibility at reduced tem- 

 peratures now indicated does not explain the absence of proto-hydrogen, 

 oxygen, and nitrogen from the spectra of the sun and nebulae, the 

 metals present in, and the absence of quartz from, meteorites, and the 

 similarity of the gaseous products obtained from them and metals, 

 native and other, in vacuo at high temperatures. 



I have finally to express my obligations to those who have aided 

 me in the present inquiry. For some of the metals used I am indebted 

 to Mr. George Matthey, F.R.S., who has kindly placed the resources of 

 his establishment so entirely at my disposal that I feel it is impossible 

 to thank him sufficiently. For the determination of wave-lengths and 

 the correspondence of terrestrial and stellar lines, Mr. Baxandall is 

 responsible, while Mr. Fowler has assisted in the determination of the 

 various stellar groups. The photographs of the enhanced lines obtained 

 by the use of the Spottiswoode coil have been taken by Mr. Butler. 

 The actual construction of the map from the available photographs 

 has devolved upon Mr. Baxandall, Dr. Lockyer co-operating in the 

 case of stars of the highest temperature. 



