18 Sir WiUiam Thomson [Jan. 21, 



tlie cases we have been now considering as examples; or it must 

 have been some number more than two — some finite number — at 

 the most the number of atoms in the sun's present mass, a finite 

 number (which may probably enough be something between 4 x 10^'^ 

 and 140 x 10^^) as easily understood and imagined as number 4 or 

 140. The immediate antecedent to incandescence may have been the 

 whole constituents in the extreme condition of subdivision — that is 

 to say, in the condition of separate atoms ; or it may have been 

 any smaller number of groups of atoms making minute crystals or 

 groups of crystals — snow-flakes of matter, as it were ; or it may have 

 been lumps of matter like a macadamising stone ; or like this 

 stone* (Fig. 1), which you might mistake for a macadamising 



Fig. 1. 



, 5 centimetres . 



stone, and which was actually travelling through space till it fell on 

 the earth at Possil, in the neighbourhood of Glasgow, on April 5, 

 1804 ; or like that * (Fig. 2) which was found in the Desert of 

 Atacama, in South America, and is believed to have fallen 

 there from the sky — a fragment made up of iron and stone, which 



* These three meteorites are in the possession of the Hunterian Museum of 

 the University of Glasgow, and the woodcuts, Figs. 1, 2, and 3, have been 

 executed from the actual specimens kindly lent for that purpose by the keeper of 

 the museum, Professor Young. The specimen represented by Fig. 1 is contained 

 in the Hunterian collection, that by Fig. 2 in the Eck collection, and that by 

 Fig. 3 in the Lanfine collection — the scale of dimensions is shown for each. It 

 may be remarked that Fig. 2 represents a section of the meteorite taken in the 

 plane of the longest rectangular axes; the bright markings being large and 

 well-formed crystals of olivine, embedded in a matrix of iron. In Fig. 3 is 

 depicted the beautiful Widmanstatten marking characteristic of all meteoric iron, 

 and so well shown in the well-known Lenarto meteorite. 



